Solid-State Modeling Of The Terahertz Spectrum Of The High Explosive HMX
Welcoming 2006 with a bang. Or the molecular foundations for a bang, anyway. In the Journal of Physical Chemistry A (J. Phys. Chem. A 2006, 110, 1951). The experimental results were provided by TeraView, Ltd., solid-state calculations with DMol3 from Accelrys.
D. G. Allis, D. A. Prokhorova, and T. M. Korter
Abstract: The experimental solid-state terahertz (THz) spectrum (3-120 cm-1) of the beta-crystal form of the high explosive octahydro – 1,3,5,7 – tetranitro – 1,3,5,7 – tetrazocine (HMX) has been analyzed using solid-state density functional theory calculations. Various density functionals (both generalized gradient approximation and local density approximation) are compared in terms of their abilities to reproduce the experimentally observed solid-state structure and low-frequency vibrational motions. Good-to-excellent agreement between solid-state theory and experiment can be achieved in the THz region where isolated-molecule calculations fail to reproduce the observed spectral features, demonstrating a clear limitation of using isolated-molecule calculations for the assignment of THz frequency motions in molecular solids. The deficiency of isolated-molecule calculations is traced to modification of the molecular structure in the solid state through crystal packing effects and the formation of weak C-H…O hydrogen bonds.
February 3rd, 2006 00:52
[...] Six years of dues finally paid off this week as my new favorite editor Gilbert Chin selected the HMX THz DFT paper (posted below) that just came out in J. Phys. Chem. A as an AOK Editor’s Choice in Science magazine. So far as I know, this page is publicly accessible without subscription, although I do encourage you to join the A.A.A.S. and help us all push this boulder that is U.S. research up the North face of Mt. Everest. [...]
April 25th, 2006 08:37
[...] Why fret the lowly Mulliken analysis? While Mulliken charge/population analyses are famously basis set-dependent and have been shown to be of limited use in describing various chemical processes (see, for instance, this interesting article at http://www.aip.org, the very handy little reference at http://www.chamotlabs.com, a CCL discussion/post of it at http://www.ccl.net and, for good measure, my example for the molecular explosive PETN in a previous post), the difference-dipole method that was used in the solid-state analyses of HMX and PETN yielded the best answers with the crystal cell dipole moments generated from Mulliken charges (perhaps a case where they excel, but time and more calculations will tell). If, for some reason, you find yourself wanting to try this approach for predicting infrared intensities for the reproduction of THz spectra, use the previous version of DMol3 (3.2) or request an older copy of the Materials Studio CD and use the same license. [...]
August 16th, 2006 08:10
[...] In press, available from the International Journal of High Speed Electronics and Systems. In continuing efforts to pull every bit of useful information out of the solid-state density functional theory calculations run for the terahertz modeling of all the molecular explosives spectra Teraview, Ltd. could get their hands on, this IJHSES paper expands greatly on the SPIE paper posted previously (available here), looking at the dependences on parameter selection in the reproductions of THz spectra for HMX and PETN. Not the assignments of the two spectra (the HMX was performed as posted below, the PETN is as posted above), but the variation in simulation quality as a function of the treatment of the electronic structure (functionals, basis sets, integration grid sizes). I’m in complete agreement with Kieron Burke (well, it’s a fact, not an opinion, so what choice do I have) that “Density functional theory is a completely different, formally rigorous, way of approaching any interacting problem, by mapping it exactly to a much easier-to-solve non-interacting problem.” The problem, to quantum chemists, is the empirical methodology used to develop density functionals. The fact that programs like GAMESS and Gaussian offers 30-or-so density functionals should clue the user in that the agreement between theory and experiment may be as much due to the choice of density functional as the quality of the basis set for the chemical question being addressed. Empirical methods mean never having to say you’re sorry. [...]
August 16th, 2006 08:43
[...] In press, available from ChemPhysChem. Wasting no time putting it all on the site. This is the sister paper to the solid-state density functional theory (DFT) paper on HMX from J. Phys. Chem. A (including the complete assignment of modes and motions of the PETN crystal using the DMol3 program) that made pretty good press. As for the determined agreement with experiment in these projects, two points certainly make a line. At the very least, the futility of isolated-molecule calculations for the assignment of crystalline THz spectra is certainly evident in the analysis. There’s quite a bit more work to be had for this project, of course. The good chaps at Teraview, Ltd., the supplier of the spectra, have recently gone cryogenic, which is going to do wonders for the theoretical assignments. D. G. Allis and T. M. KorterAbstract: The experimental solid-state terahertz (THz) spectrum (3 to 120 cm-1) of the high explosive pentaerythritol tetranitrate (PETN, C5H6N4O12) has been modeled using solid-state density functional theory (DFT) calculations. Solid-state DFT employing the BP density functional is in best qualitative agreement with the features in the previously reported THz spectrum. The crystal environment of PETN includes numerous intermolecular hydrogen-bonding interactions that contribute to large (up to 80 cm-1) calculated shifts in molecular normal mode positions in the solid-state. Comparison of the isolated-molecule and solid-state normal mode calculations for a series of density functionals reveals the extent to which the inclusion of crystal packing interactions and the relative motions between molecules are required for correctly reproducing the vibrational structure of solid-state THz spectra. The THz structure below 120 cm-1 is a combination of both intermolecular (relative rotations and translations) and intramolecular (torsions, large amplitude motions) vibrational motions. Vibrational mode analyses indicate that the first major feature (67.2 cm-1) in the PETN THz spectrum contains all of the optical rotational and translational cell modes and no internal (molecular) vibrational modes. [...]
August 17th, 2006 12:14
[...] This is a real nice summary by Michael Francis at Accelrys about the use of solid-state density functional theory (DFT) in the assignment of the HMX THz spectrum published in J. Phys. Chem. A earlier this year. Also, I’m on record again extolling the virtues of DMol3, having now used it in a number of inelastic neutron scattering studies, this crazy terahertz business, and for (an upcoming) looking at the crystal environments of polyoxometalates (which I wouldn’t have even considered as early as two years ago given the state of computers way back when). Three more papers on the subject have come out since this case study was presented, all of which are linked and available in the terahertz spectroscopy category of this site. [...]
September 1st, 2006 11:44
[...] An important aspect of any solid-state density functional theory analysis of molecular crystals, such as the high explosives HMX and PETN onsidered in some previous posts, is identifying just how those molecules change in the solid-state. This is accomplished by simply performing an isolated-molecule calculation at the same level of theory on a single molecule to identify where the structural and vibrational differences from the solid-state results lie, the vibrational aspect being the focus of the terahertz studies considered in those previous HMX and PETN posts. The comparisons of the low-frequency isolated-molecule and solid-state vibrational modes make obvious (1) how much the external vibrational modes BETWEEN molecules (the rotational and translational modes ignored in isolated-molecule normal mode analyses) contribute to structure in solid-state THz spectra and (2) how much the crystal environment affects the geometries of single molecules through electrostatic interactions (this is obvious in periodic hydrogen-bonding systems, but even non-polar molecules like dodecahedrane are deformed from their icosahedral symmetry due to weak intermolecular interactions between neighboring molecules in its crystal packing environment). [...]
February 21st, 2010 22:41
[...] edition of the journal (having missed it by three intermediate articles with the Cs2B12H12 and HMX papers back in 2006 (you’d keep track, too). A fortuitous overlap of scheduled defense dates [...]