In the event anyone is trying to perform some type of post-SCF Mulliken charge analysis in DMol3 4.0, be warned! Or, perhaps, don’t sweat the crazy results. Much to my dismay (after installing 10 new copies), there is a reported bug (reference id=06090p1rq01 to Accelrys) concerning the reporting of Mulliken charges in DMol3 runs where, as far as I’ve seen, the first 10 to 12 Mulliken charges appear reasonable and the remaining charges are the same small number. Hirshfeld charges appear unaffected (and are consistent with the results from previous DMol3 versions).
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 www.aip.org, the very handy little reference at www.chamotlabs.com, a CCL discussion/post of it at 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.
In press, available from the SPIE, the International Society For Optical Engineering (don’t ask me). One of the most interesting pieces of information to come from this study, besides discovering the SPIE’s restriction to grey scale images in this day and age, is just how insensitive to basis set the Hirshfeld partitioning method is (across the DN, DND, DNP DMol3 basis sets) and just how sensitive the Mulliken partitioning method is. This is, of course, not new to quantum chemistry, but it is of interest to note that the same basic trends that apply to Gaussian-type basis sets also apply to numerical basis sets (trends, interpretive limitations, what’s the difference?). The plot below shows the relative charges on each symmetry-unique atom in PETN. These trends have significant impacts on the calculation of THz intensities by difference-dipole methods, as is discussed in detail in this article text (I have glossed over quite a bit of analysis and report the charge findings for purely academic reasons).
Damian G. Allis, Darya A. Prokhorova, Anna M. Fedor, and Timothy M. Korter
Abstract: This paper focuses on the establishment of methodologies for the successful application of first principles theoretical analyses in the understanding of the terahertz spectroscopy of molecular solids, particularly high-energy materials. A solid-state density functional theoretical study has been performed on the high explosive pentaerythritol tetranitrate (PETN) in order to address the relationships between the choice of computational parameters and the predictions of molecular and solid-state properties, such as molecular geometries, intermolecular interactions within the crystal cell, charge distributions, and solid-state vibrations in the terahertz (3 to 200 cm-1) region. This investigation has lead to the conclusion that the BP functional has the best overall performance and the choice of basis set is the most critical theoretical variable. Varying other parameters such as grid size, orbital cut-off, and convergence criteria lead to negligible differences among the calculations.
More free press concerning the HMX paper in J.Phys.Chem. A. in the form of “THz spectrum yields to theory” in the Science and Technology Concentrates section of C&E News (February 6, 2006, Volume 84, Number 6, p. 25). ACS-only accessible if you’re not on a campus with a subscription. Incidentally, the editor who made us look good, Mitch Jacoby, also received some free press lately doing something I wish I was doing more often. Check’em out at www.thewayjewsrock.com.