DNA-Specific (But Generally Applicable) AMBER With GROMACS 3.3.x: Installation And Notes

The following is the full procedure for installing the AMBER force field port for GROMACS (AMBER-in-GROMACS, AMBER-with-GROMACS, AMBER-on-GROMACS, whatever you want to call it) developed by Eric Sorin at California State University, Long Beach, providing a bit more depth in the installation process (specifically for GROMACS 3.3.x) and a few modified GROMACS files.

As brief background, AMBER (Assisted Model Building and Energy Refinement) is one of THE dominant molecular mechanics/molecular dynamics (MM/MD) force fields used today in biochemical simulations. The motivation for this page (my installing AMBER for use in GROMACS) stems from the current Nanorex focus on Structural DNA Nanotechnology (SDN) modeling, for which we’re working on a reduced model force field for large-structure energy minimizations and, importantly, integrating the GROMACS MM/MD package for use via our CAD interface. You can read more about this in the poster presented at FNANO08 this past April. As a force field validated for DNA simulations, AMBER meets our needs of performing atomistic simulations on DNA nanostructures. While NAMD is also a possibility for DNA simulations, GROMACS meets Nanorex’s open source needs.

Needless to say, finding that Sorin and co-workers had ported AMBER to GROMACS was a wonderful discovery (and that this same port appears to be driving the Folding@Home project). The installation directions on the AMBER-on-GROMACS website are good, but they don’t mention a few important steps that I spent no small amount of time trying to figure out (and I’m definitely not complaining. 99% of the work was the porting, which has been graciously handed to the GROMACS community on a silver platter). Below is the complete set of steps required to get, as the first example, the Dickerson.pdb sample file provided with the porting files to energy minimize and MD correctly. Issues related to DNA-based simulations not using the Dickerson.pdb file will be covered in another post.

Dickerson DNA

QuteMol rendering of Dickerson.pdb (from Drew, H. R., Wing, R. M., Takano, T., Broka, C., Tanaka, S., Itakura, K. & Dickerson, R. E. (1981). Structure of a B-DNA dodecamer: conformation and dynamics. Proc. Natl. Acad. Sci. USA 78, 2179-2183. Protein Data Bank structure 1BNA).

Install/compile a custom installation of GROMACS

The AMBER port to GROMACS does not require modification of the GROMACS code but does require a few changes to force field text files. In the interest of remembering what was changed and what wasn’t, I recommend a custom compilation of GROMACS or, at the very least, the installation of another copy of GROMACS you can modify the top directory of without risking changes to an already working GROMACS version.

If you don’t know how to compile your own version of GROMACS, I recommend taking a look at Compiling Single-Precision And Double-Precision GROMACS 3.3.3 With OpenMPI 1.2.6 Under OSX 10.5 (Leopard).

My installation directory is /usr/local/gromacs333-amber (which I will refer to here as /ULG-A/ and all directory calls will be to this. Obviously, change all directory calls to match yours.

Download GROMACS-Ported AMBER force field files

I’m running a double-precision MPI version of GROMACS 3.3.3, meaning I’m using the GROMACS-ported AMBER force field files for GROMACS 3.3.1 (May 2006). And get the version “with pdfs.” If you’re going to be doing these calculations, you should know where the math come from!

Copy vdwradii.dat and aminoacids-NA.dat into /usr/local/gromacs-amber/share/gromacs/top

You will need to be logged in as root to do this (sudo cp FILENAME /ULG-A/top). The AMBER port file vdwradii.dat differs from the vdwradii.dat file in the /ULG-A/top directory of the GROMACS install by the addition of 5 lines:

???  P     0.15
???  LP1   0
???  LP2   0
LYSH MNZ1  0
LYSH MNZ2  0

With no removals or number changes.

The aminoacids.dat file is a little different. In order to use pdb2gmx to generate nucleic acid topology files, GROMACS requires the list of residue codes that reside in aminoacids-NA.dat. So far as I can tell, using aminoacids-NA.dat instead of aminoacids.dat does not change the handling of amino acids (why two files exist for a reason other than amino acid-centric organization is beyond my pay scale), so we’ll be using aminoacids-NA.dat exclusively.

Move/delete/rename aminoacids.dat and rename aminoacids-NA.dat to aminoacids.dat

The aminoacids-NA.dat file includes 32 additional residue codes, accounting for the 3′ (includes terminal H atom), 5′ (included terminal H atom), in-strand (nucleic acid repeat unit) and molecule (individual hydrogen-terminated sugar and nucleic acid) topology information codes for DNA and RNA found in the .rtp files

Copy the ffamber files of interest into the /ULG-A/top directory

If you’re doing this in Linux or OSX, you might as well move all of the files you want installed into a single directory and su cp * /ULG-A/top. Note that the .itp and .gro files in your ffamber_v3.3.1 (for the 3.3.x GROMACS) are the same for all of the AMBER force field flavors you can install. Further, all of the ffamber* files have unique names (94, 99, 03, etc.), so you can install all the force field files into /ULG-A/top and use whichever.

Modify /ULG-A/top/FF.dat to reflect the new force field files

The FF.dat file as installed by GROMACS looks like the following:

9
ffG43a1  GROMOS96 43a1 force field
ffG43b1  GROMOS96 43b1 vacuum force field
ffG43a2  GROMOS96 43a2 force field (improved alkane dihedrals)
ffG45a3  GROMOS96 45a3 force field (Schuler JCC 2001 22 1205)
ffG53a5  GROMOS96 53a5 force field (JCC 2004 vol 25 pag 1656)
ffG53a6  GROMOS96 53a6 force field (JCC 2004 vol 25 pag 1656)
ffoplsaa OPLS-AA/L all-atom force field (2001 aminoacid dihedrals)
ffencadv Encad all-atom force field, using scaled-down vacuum charges
ffencads Encad all-atom force field, using full solvent charges    

To include the AMBER force field files (so that these separate files can be called during pdb2gmx), we modify the FF.dat file by (1) incrementing the total number of force fields and (2) adding the force field code and text description to this file. The new FF.dat file will look like this if you added all of the ffamber force field files:

17
ffG43a1  GROMOS96 43a1 force field
ffG43b1  GROMOS96 43b1 vacuum force field
ffG43a2  GROMOS96 43a2 force field (improved alkane dihedrals)
ffG45a3  GROMOS96 45a3 force field (Schuler JCC 2001 22 1205)
ffG53a5  GROMOS96 53a5 force field (JCC 2004 vol 25 pag 1656)
ffG53a6  GROMOS96 53a6 force field (JCC 2004 vol 25 pag 1656)
ffoplsaa OPLS-AA/L all-atom force field (2001 aminoacid dihedrals)
ffencadv Encad all-atom force field, using scaled-down vacuum charges
ffencads Encad all-atom force field, using full solvent charges
ffamber94 AMBER94 Cornell et al. (1995), JACS 117, 5179-5197
ffamber96 AMBER96 Kollman (1996), Acc. Chem. Res. 29, 461-469
ffamberGS AMBER99GS Garcia & Sanbonmatsu (2002), PNAS 99, 2782-2787
ffamberGSs AMBER99GSs Nymeyer & Garcia (2003) PNAS 100, 13934-13939
ffamber99 AMBER99 Wang et al. (2000), J. Comp. Chem. 21, 1049-1074
ffamber99p AMBER99p Sorin & Pande (2005). Biophys. J. 88(4), 2472-2493
ffamber99SB AMBER99sb Hornak et. al (2006). Proteins 65, 712-725
ffamber03 AMBER03 Duan et al. (2003), J. Comp. Chem. 24, 1999-2012

Add and increment numbers as appropriate for the force field you want to use. You can download the complete FF.dat at the following link and comment:

Download amber_FF.txt, delete “amber_”, change “.txt” to “.dat”, place in /ULG-A/top

source /UGL-A/bin/GMXRC

The GMXRC file contains path-dependent settings for your shell.

In theory, you’re all done. At least, this is the end of the installation process according to the official AMBER-on-GROMACS list. The IMPORTANT NOTES section contains relevant information but does not complete the installation. The additional steps are provided below.

Modify the /ULG-A/top/spc.itp file

If you’re running through a complete energy minimization calculation with the website installation followed, your first source of error (hopefully) comes in the form of…

Program grompp_amber, VERSION 3.3.3
Source code file: toppush.c, line: 1193

Fatal error:
[ file "/usr/local/gromacs333_amber/share/gromacs/top/spc.itp", line 32 ]:
Atom index (1) in bonds out of bounds (1-0).
This probably means that you have inserted topology section "bonds"
in a part belonging to a different molecule than you intended to.
In that case move the "bonds" section to the right molecule.

The origin of this error is the lack of amberXX water parameters in the spc.itp file (nothing directly to do with the reported GROMACS error). The fix for this is straightforward, simply modifying the spc.itp file in the /ULG-A/top directory as follows below. The original spc.itp file below…

[ moleculetype ]
; molname       nrexcl
SOL             2

[ atoms ]
;   nr   type  resnr residue  atom   cgnr     charge       mass
#ifdef _FF_GROMACS
1     OW      1    SOL     OW      1      -0.82
2     HW      1    SOL    HW1      1       0.41
3     HW      1    SOL    HW2      1       0.41
#endif
#ifdef _FF_GROMOS96
#ifdef HEAVY_H
1     OW      1    SOL     OW      1      -0.82    9.95140
2      H      1    SOL    HW1      1       0.41    4.03200
3      H      1    SOL    HW2      1       0.41    4.03200
#else
1     OW      1    SOL     OW      1      -0.82   15.99940
2      H      1    SOL    HW1      1       0.41    1.00800
3      H      1    SOL    HW2      1       0.41    1.00800
#endif
#endif
#ifdef _FF_OPLS
1  opls_116   1    SOL     OW      1      -0.82
2  opls_117   1    SOL    HW1      1       0.41
3  opls_117   1    SOL    HW2      1       0.41
#endif

#ifdef FLEXIBLE
[ bonds ]
; i     j       funct   length  force.c.
1       2       1       0.1     345000  0.1     345000
1       3       1       0.1     345000  0.1     345000

[angles ]
; i     j       k       funct   angle   force.c.
2       1       3       1       109.47  383     109.47  383
#else
[ settles ]
; OW    funct   doh     dhh
1       1       0.1     0.16330

[ exclusions ]
1       2       3
2       1       3
3       1       2
#endif

is modified to this…

[ moleculetype ]
; molname       nrexcl
SOL             2

[ atoms ]
;   nr   type  resnr residue  atom   cgnr     charge       mass
#ifdef _FF_GROMACS
1     OW      1    SOL     OW      1      -0.82
2     HW      1    SOL    HW1      1       0.41
3     HW      1    SOL    HW2      1       0.41
#endif
#ifdef _FF_GROMOS96
#ifdef HEAVY_H
1     OW      1    SOL     OW      1      -0.82    9.95140
2      H      1    SOL    HW1      1       0.41    4.03200
3      H      1    SOL    HW2      1       0.41    4.03200
#else
1     OW      1    SOL     OW      1      -0.82   15.99940
2      H      1    SOL    HW1      1       0.41    1.00800
3      H      1    SOL    HW2      1       0.41    1.00800
#endif
#endif
#ifdef _FF_OPLS
1  opls_116   1    SOL     OW      1      -0.82
2  opls_117   1    SOL    HW1      1       0.41
3  opls_117   1    SOL    HW2      1       0.41
#endif

#ifdef _FF_AMBER94
; also applies to FF_AMBER96, FF_AMBERGS, and FF_AMBERGSs
1  amber94_42   1  SOL     OW      1      -0.82  15.99940
2  amber94_27   1  SOL    HW1      1       0.41   1.00800
3  amber94_27   1  SOL    HW2      1       0.41   1.00800
#endif

#ifdef _FF_AMBER99
; also applies to FF_AMBER99P, FF_AMBER99SB, FF_AMBER03
1  amber99_54   1  SOL     OW      1      -0.82  15.99940
2  amber99_55   1  SOL    HW1      1       0.41   1.00800
3  amber99_55   1  SOL    HW2      1       0.41   1.00800
#endif



#ifdef FLEXIBLE
[ bonds ]
; i     j       funct   length  force.c.
1       2       1       0.1     345000  0.1     345000
1       3       1       0.1     345000  0.1     345000

[ angles ]
; i     j       k       funct   angle   force.c.
2       1       3       1       109.47  383     109.47  383
#else
[ settles ]
; OW    funct   doh     dhh
1       1       0.1     0.16330

[ exclusions ]
1       2       3
2       1       3
3       1       2
#endif

This FFAMBER-included spc.itp file is downloadable in the following form and comment:

Download amber_spc.txt, delete “amber_” change “.txt” to “.itp”, place in /ULG-A/top

At this point, pdb2gmx, editconf, genbox, grompp, and genion should all work without error. The next problem comes with the post-genion grompp, which can’t find ion information…

Modify ions.itp

Similar to the spc.itp error, the error that comes up with the inclusion of counterions confounds the mind because everything seems to be in place.

Program grompp_amber, VERSION 3.3.3
Source code file: toppush.c, line: 1396

Fatal error:
No such moleculetype Na

With Na being whatever counterion you’re trying. The AMBER-on-GROMACS website reports the following for AMBER ion inclusion:

Our AMBER ports include common ion definitions, which are listed in the ffamber*.rtp files (just below the TIP water models). This allows the AMBER ports to be used without modification or use of the GROMACS ions.itp file. At the moment these include Cl- , IB+, Na+, K+ , Rb+, Cs+, Li+ , Ca2+, Mg2+, Zn2+, Sr2+, and Ba2+. If your pdb file has ions present and pdb2gmx does not properly convert those ions, please check the atom and residue name of your ions and rename them if necessary to agree with the .rtp file. If you are using ion-related GROMACS tools, such as genion, you will need to enter the AMBER ion definition to the ions.itp file in the “top” directory of the GROMACS distribution.

The bases for the ion use are in the .rtp files, but the implementation, at least for GROMACS 3.3.3 and possibly earlier versions, is not as found in these files.

The proper format for both the FFAMBER_94 and FFAMBER_99 ion types in ions.itp is as follows:


#ifdef _FF_AMBER94

[ moleculetype ]
; molname       nrexcl
Na              1
[ atoms ]

; id    at type         res nr  residu name     at name  cgnr   charge    mass
1       amber94_31      1       Na              Na       1      1         22.99000
#endif

#ifdef _FF_AMBER99

[ moleculetype ]
; molname       nrexcl
Na              1
[ atoms ]

; id    at type         res nr  residu name     at name  cgnr   charge    mass
1       amber99_31      1       Na              Na       1      1         22.9900
#endif

This is the format used for the FF_GROMOS96 ions with one change. The atom type for the FF_AMBERXX parameters is NOT the atom label, but instead the AMBER force field label. In the case of Na, this is amber94_31 (for AMBER94) and amber99_31 (for AMBER99). Note that the mass must be specified (you have to scroll down the ions.itp file to see FF_GROMOS96. FF_GROMACS does not require mass specification).

For quick reference, here are the following [ atoms ] specifications for FF_AMBER94 and FF_AMBER_99.

#ifdef _FF_AMBER94
; id    at type         res nr  residu name     at name  cgnr   charge    mass
1       amber94_32      1       IB              IB       1      1         131.00000
1       amber94_56      1       Li              Li       1      1         6.94000
1       amber94_31      1       Na              Na       1      1         22.99000
1       amber94_51      1       K               K        1      1         39.10000
1       amber94_52      1       Rb              Rb       1      1         85.47000
1       amber94_53      1       Cs              Cs       1      1         132.90000
1       amber94_33      1       Mg              Mg       1      2         24.30500
1       amber94_15      1       Ca              Ca       1      2         40.08000
1       amber94_57      1       Zn              Zn       1      2         65.40000
1       amber94_58      1       Sr              Sr       1      2         87.62000
1       amber94_59      1       Ba              Ba       1      2         137.33000
1       amber94_30      1       Cl              Cl       1      -1        35.45000
#endif

#ifdef _FF_AMBER99
; id    at type         res nr  residu name     at name  cgnr   charge    mass
1       amber99_32      1       IB              IB       1      1         131.00000
1       amber99_56      1       Li              Li       1      1         6.94000
1       amber99_31      1       Na              Na       1      1         22.99000
1       amber99_51      1       K               K        1      1         39.10000
1       amber99_52      1       Rb              Rb       1      1         85.47000
1       amber99_53      1       Cs              Cs       1      1         132.90000
1       amber99_33      1       Mg              Mg       1      2         24.30500
1       amber99_15      1       Ca              Ca       1      2         40.08000
1       amber99_57      1       Zn              Zn       1      2         65.40000
1       amber99_58      1       Sr              Sr       1      2         87.62000
1       amber99_59      1       Ba              Ba       1      2         137.33000
1       amber99_30      1       Cl              Cl       1      -1        35.45000
#endif

To save yourself plenty of trouble, download the ions.itp file with all of the ion parameters in the form of the following link:

Download amber_ions.txt, delete “amber_”, change “.txt” to “.itp”, place in /ULG-A/top

With the ions.itp file complete, your GROMACS energy minimizations and dynamics simulations of amino acid and nucleic acid structures should go without hitch.

My thanks to Alan Wilter S. da Silva, D.Sc. – CCPN Research Associate, Department of Biochemistry, University of Cambridge, for catching a formatting error in the first version of the amber_ions.itp file.

If you find problems, questions, concerns, incompatibilities, etc., please let me know by setting up a user account and posting a comment in this post or email me so I can post whatever you might find.

And don’t forget to cite ffAMBER if you use it!

Sorin & Pande (2005). Biophys. J. 88(4), 2472-2493

chemistry.csulb.edu/ffamber
www.gromacs.org
chemistry.csulb.edu/esorin
chemistry.csulb.edu
amber.scripps.edu
www.nanorex.com
en.wikipedia.org/wiki/DNA_nanotechnology
www.somewhereville.com/rescv/fnano08_2008_poster.jpg
www.cs.duke.edu/~reif/FNANO
www.ks.uiuc.edu/Research/namd
en.wikipedia.org/wiki/Open_source
folding.stanford.edu
qutemol.sourceforge.net
www.pdb.org
www.pdb.org/pdb/explore/explore.do?structureId=1BNA
www.linux.org
www.apple.com/macosx
www.gromacs.org/documentation/reference/online/pdb2gmx.html

Oral Insulin Delivery Cover Image (And Associated Syracuse Research Article) in ChemMedChem

You’ve heard about it, you’ve read about it, you’ve seen it on color TV, you’ve even seen it streamed. The cover story in this month’s issue of ChemMedChem is a communication by members and collaborators of the Robert Doyle Group here at Syracuse University. The report describes the B12/TCII-based uptake of insulin, a process that occurs via the ingestion of a B12-insulin conjugate. In case you missed that, the delivery is oral, not by needle. For those of us that pass out at anything needle-related at about the time that the alcohol wipe is opened, that’s a positive step forward for getting rid of any syringe-related medicine altogether.

full image

With the cover story comes the cover image shown above, a structure calculation on the insulin-B12/TCII complex. The bases for this structure can be found in the Protein Data Bank, including the TCII-B12 complex reported in PDB entry 2BB5 (the only hack in the structure calculation involved the replacement of the cobalt for iron to use already available bond parameters) and the insulin structure reported in PDB entry 1ZNI. The covalent attachment of the insulin to B12 can be found in the article. Structure manipulation was performed with a combination of NanoEngineer-1 and VMD, VMD being included in the mix in order to generate the ribbon renderings of the insulin and TCII protein backbones. As for the accuracy of the calculation, time and a synchrotron X-ray source will tell.

For much more information and numerous links to new stories related to the research in the article, I direct you to the group website of Robert Doyle and the various links to news stories available in his departmental publication list.

chemmedchem cover
From ChemMedChem. Click HERE to go to the article.

From the website:

Cover Picture: Vitamin B12 as a Carrier for the Oral Delivery of Insulin (ChemMedChem 12/2007). The cover picture shows an orally active, glucose-lowering vitamin B12-insulin conjugate bound to the B12 uptake protein transcobalamin II (TCII). The inset shows a close-up view of the TCII binding pocket. (Insulin is in red; vitamin B12 is in bright yellow.) For details, see the Communication by T. J. Fairchild, R. P. Doyle, et al. on p. 1717 ff.

www3.interscience.wiley.com/journal/110485305/home
chemistry.syr.edu/faculty/doyle.html
www.syr.edu
www3.interscience.wiley.com/cgi-bin/abstract/117354616/ABSTRACT?CRETRY=1&SRETRY=0
en.wikipedia.org/wiki/B12
en.wikipedia.org/wiki/Insulin
en.wikipedia.org/wiki/Transcobalamin
www3.interscience.wiley.com/journal/117354609/graphissue
www.rcsb.org/pdb
www.rcsb.org/pdb/explore.do?structureId=2BB5
www.rcsb.org/pdb/explore.do?structureId=1ZNI
www.nanorex.com
www.ks.uiuc.edu/Research/vmd
en.wikipedia.org/wiki/Synchrotron
chemistry.syr.edu/faculty/doyle_group/index.html
chemistry.syr.edu/faculty/doyle.html#pubs

Extension Of The Single Amino Acid Chelate Concept (SAAC) To Bifunctional Biotin Analogues For Complexation Of The M(CO)3+1 Core (M = Tc And Re): Syntheses, Characterization, Biotinidase Stability And Avidin Binding

In press, available from the journal Bioconjugate Chemistry. The modeling study for the avidinbiotin structure and the biotin derivatives were completed with the molecular dynamics program NAMD on a Dual G4/450 loaned to me from Apple for development work, for which I am grateful (I’ve performed molecular dynamics simulations with the Walrus). I did manage to smoke the motherboard during this experience, for which I apologize. Given the state of the machine after the autopsy, I’m hoping no one (especially Eric Zelman!) asks for it back, even when I’m 64.

I made mention of the reasons for some of this work in an interview I did for nanotech.biz, completely unrelate to the other content, in case anyone wants some background.

Shelly James, Kevin P. Maresca, Damian G. Allis, John F. Valliant, William Eckelman, John W. Babich, and Jon Zubieta

Abstract: Biotin and avidin form one of the most stable complexes known (KD = 10-15M-1) making this pairing attractive for a variety of biomedical applications including targeted radiotherapy. In this application one of the pair is attached to a targeting molecule while the other is subsequently used to deliver a radionuclide for imaging and/or therapeutic applications. Recently we reported a new single amino acid chelate (SAAC) capable of forming robust complexes with Tc(CO)3 or Re(CO)3 cores. We describe here the application of SAAC analogs for the development of a series of novel radiolabeled biotin derivatives capable of forming robust complexes with both Tc and Re. Compounds were prepared through varying modification of the free carboxylic acid group of biotin. Each 99mTc complex of SAAC-biotin was studied for their ability to bind avidin, susceptibility to biotinidase and specificity for avidin in an in vivo avidin-containing tumor model. The radiochemical stability of the 99mTc(CO)3 complexes was also investigated by challenging each 99mTc-complex with large molar excesses of cysteine and histidine at elevated temperature. All compounds were radiochemically stable for greater than 24 hours at elevated temperature in the presence of histidine and cysteine. Both [99mTc(CO)3(L6)]+1 [TcL6; L6 = biotinyl- amido- propyl- N,N- (dipicolyl)- amine] and [99mTc(CO)3(L12a)]+1 (TcL12; L12 = N,N-(dipicolyl)- biotin- amido- Boc- lysine; TcL12a; L12a = N,N- (dipicolyl)- biotin- amide- lysine) readily bound to avidin whereas [99mTc(CO)3(L9)]+1 [TcL9; L9 = N,N- (dipicolyl)- biotin- amine] demonstrated minimal specific binding. TcL6 and TcL9 were resistant to biotinidase cleavage while TcL12a, which contains a lysine linkage, was rapidly cleaved. The highest uptake in an in vivo avidin tumor model was exhibited by TcL6, followed by TcL9 and TcL12a, respectively. This is likely the result of both intact binding to avidin and resistance to circulating biotinidase. Ligand L6 is the first SAAC analogue of biotin to demonstrate potential as a radiolabeled targeting vector of biotin capable of forming robust radiochemical complexes with both 99mTc and rhenium radionuclides.Computational simulations were performed to assess biotin-derivative accommodation within the binding site of the avidin. These calculations demonstrate that deformation of the surface domain of the binding pocket can occur to accommodate the transition metal-biotin derivatives with negligible changes to the inner-β-barrel, the region most responsible for binding and retaining biotin and its derivatives.

P.S. This publication is also of some use for explaining the series of images on the current departmental brochure for the Syracuse U. Chemistry Department. Steric interactions affect the local geometries of protein binding pockets. And a good thing, too.


Click on the image for a larger version.

www.apple.com
www.applecorps.com
chemistry.syr.edu/faculty/zubieta.html
www.molecularinsight.com
www.chemistry.mcmaster.ca/people/faculty/valliant/index.html
pubs.acs.org/journals/bcches/index.html
www.ks.uiuc.edu/Research/namd/