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This is the first reported use of a hybrid method involving density functional theory (DFT) and symmetry-adapted perturbation theory (SAPT) to calculate intermolecular interactions. This work was stimulated by the reported failures of supermolecular DFT calculations to adequately predict intermolecular (and interatomic) interactions, particularly of the van der Waals type. The goals are to develop a hybrid scheme that will calculate intermolecular interaction energies accurately and in a computationally efficient fashion, while including the benefits of the energy decomposition provided by SAPT. The computational savings result from replacing the costly perturbation theory treatment with DFT, which should include the intramolecular correlation effects on the intermolecular interaction energies. The accuracy of this new hybrid approach (labeled SAPT(DFT)) is evaluated by comparisons with higher level calculations. The test cases include He2, Ar2, Ar−H2, (H2O)2, (HF)2, CO2−CH3CN, and CO2-dimethylnitramine. The new approach shows mixed results concerning the accuracy of interaction energies. SAPT(DFT) correctly predicts all the qualitative trends in binding energies for all test cases. This is particularly encouraging in dimer systems dominated by dispersive interactions where supermolecular DFT fails to predict binding. In addition, the method achieves a drastic reduction (a factor of at least 100) in computational time over the higher level calculations often used to predict these forces. With respect to quantitative accuracy, this initial hybrid scheme, using the very popular exchange-correlation functional B3LYP, overestimates the second-order energy components (e.g., induction and dispersion terms) for all of the test cases, and subsequently overestimates the total interaction energy for all dimer systems except those heavily dominated by the electrotstatic interactions. The SAPT energy decomposition points to the use of DFT virtual orbital eigenvalues in the second-order perturbation terms as the likely cause for this error. These results are consistent with earlier work suggesting that DFT canonical virtual orbital energies obtained from commonly used functionals are less than optimal for use in such a perturbative scheme. The first-order interaction energy terms from the SAPT(DFT) are found to be generally more accurate than the second-order terms, and agree well with the benchmark values for dimers containing molecules with a permanent electric dipole moment. These first-order terms depend only upon the occupied MO eigenvectors, and hence are not affected by the inaccuracies in the Kohn−Sham DFT virtual orbital eigenvalues. These observations encourage future studies utilizing newly reported functionals, some of which have been developed to directly address problems with DFT virtual orbital energies and the asymptotic region of the electron density.

Geometry optimizations and normal-mode analyses of three conformers of 1,3,5-trinitro-s-triazine (RDX) are performed using second-order Moller−Plesset (MP2) and nonlocal density functional theory (DFT) methods. The density functional used in this study is B3LYP. The three conformers of RDX are distinguished mainly by the arrangement of the nitro groups relative to the ring atoms of the RDX molecule. NO2 groups arranged in either pseudo-equatorial or axial positions are denoted with (E) or (A), respectively. The AAE conformer has C s symmetry and is the structure in the room-temperature stable crystal (α-RDX). The AAA and EEE conformers have C 3 v symmetry, a symmetry consistent with vapor and β-solid infrared spectra. The AAE and AAA conformers are studied at the MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-311+G** levels, and the EEE conformer is studied using the B3LYP density functional and the 6-31G* and 6-311+G** basis sets. The geometric parameters and infrared spectra of the AAA conformer are in good agreement with experimental gas-phase and β-solid data, supporting the hypotheses derived from experiment that the AAA structure is the most probable conformer in vapor-phase and β-solid RDX. The B3LYP/6-311+G** structures and simulated infrared spectra are in closest overall agreement with experimental data. The MP2/6-31G* structures and spectra are in poorest overall agreement with experiment.

First-principles pseudopotential plane wave calculations based on spin-polarized density functional theory ͑DFT͒ and the generalized gradient approximation ͑GGA͒ have been used to study the adsorption of CO molecules on the Fe͑100͒ surface. Among several possible adsorption configurations considered here, the most stable corresponds to a fourfold state in which a CO molecule is tilted relative to the surface normal by 50°. In this case, the CO bond is elongated to 1.32 Å and has a low vibrational stretching frequency of 1246 cm Ϫ1 to be compared with the experimental gas phase value of 2143 cm Ϫ1 . The adsorption energy for this state is found to vary between 46.7 and 43.8 kcal/mol depending on the choice of exchange-correlation functional used in the DFT. A total of three adsorption sites have been located, and the relative adsorption energies are E(fourfold)ϾE(twofold)ϷE(onefold) at lower surface coverage, and E(fourfold)ϾE(onefold) ϾE(twofold) at higher coverage. A similar analysis performed for the C and O atoms indicates that the adsorption at the fourfold site is the most stable among various configurations, with adsorption energies of 186 and 145 kcal/mol, respectively. Additionally, we have demonstrated the possibility that a C atom embeds into the lattice in a twofold, bridgelike configuration with an adsorption energy of 154 kcal/mol. The minimum energy pathways for the surface diffusion of a CO molecule between selected pairs of local minima indicate that the barriers for these processes are generally quite small with values less than 2 kcal/mol. One exception to this is the diffusion out of the most stable fourfold site, where the barrier is predicted to be around 13 kcal/mol. Finally, the barriers for dissociation of CO bound in a fourfold site have been calculated to have values in the range of 24.5-28.2 kcal/mol, supporting the experimental observation that dissociation of CO bound to the surface seems to compete with CO desorption at 440 K.

We report calculations of the mid-IR unpolarized absorption and circular dichroism spectra of 4-methyl-2-oxetanone, 6,8-dioxabicyclo[3.2.l]octane and 1,7,7-trirnethylbicyclo[2.2.l]heptan-2-one (camphor) based on harmonic force fields calculated using density functional theory (DFT). A hybrid density functional, Becke3LYP, is used. The basis set is 6-31G*. The results are in impressive agreement with experimental spectra. Calculations using the LSDA and BLYP functionals are much less successful. Our results using Becke3LYP/DFT are compared with the predictions of SCF and MP2 calculations. At the present time, the Becke3LYP/DFT methodology is clearly to be preferred in predicting mid-IR vibrational spectra.