Comparison of SCC-DFTB and NDDO-based semiempirical molecular orbital methods for organic molecules.

Extensive testing of the SCC-DFTB method has been performed, permitting direct comparison to data available for NDDO-based semiempirical methods. For 34 diverse isomerizations of neutral molecules containing the elements C, H, N, and O, the mean absolute errors (MAE) for the enthalpy changes are 2.7, 3.2, 5.0, 5.1, and 7.2 kcal/mol from PDDG/PM3, B3LYP/6-31G(d), PM3, SCC-DFTB, and AM1, respectively. A more comprehensive test was then performed by computing heats of formation for 622 neutral, closed-shell H, C, N, and O-containing molecules; the MAE of 5.8 kcal/mol for SCC-DFTB is intermediate between AM1 (6.8 kcal/mol) and PM3 (4.4 kcal/mol) and significantly higher than for PDDG/PM3 (3.2 kcal/mol). Similarly, SCC-DFTB is found to be less accurate for heats of formation of ions and radicals; however, it is more accurate for conformational energetics and intermolecular interaction energies, though none of the methods perform well for hydrogen bonds with strengths under ca. 7 kcal/mol. SCC-DFTB and the NDDO methods all reproduce MP2/cc-pVTZ molecular geometries with average errors for bond lengths, bond angles, and dihedral angles of only ca. 0.01 A, 1.5 degrees , and 3 degrees . Testing was also carried out for sulfur containing molecules; SCC-DFTB currently yields much less accurate heats of formation in this case than the NDDO-based methods due to the over-stabilization of molecules containing an SO bond.

Mindless chemistry.

Applications of an automated stochastic search procedure for locating all possible minima with a given composition are illustrated by the pentatomic molecules BCNOS, CAlSiPS, C(4)B(-), C(4)Al(-), and CBe(4)(2-), as well as by C(6)Be, the C(6)Be(2-) dianion, and C(6)H(2). All previously identified minima were reproduced, and many new structures, often with nonintuitive geometries, were found.

NO-MNDO: Reintroduction of the Overlap Matrix into MNDO.

The effect of reintroducing the overlap matrix into the secular equations for an NDDO (neglect of diatomic differential overlap)-based semiempirical molecular orbital method has been investigated. The modification is expected to improve the description of interactions between electron pairs. The idea has been tested by implementation and evaluation of a nonorthogonal version of the MNDO method (NO-MNDO) with parametrization for hydrogen, carbon, nitrogen, and oxygen. Overall, the accuracy of NO-MNDO for heats of formation is nearly identical to that for the more highly parametrized AM1 method. The mean absolute error (MAE) for heats of formation of a comprehensive set of 622 neutral, closed-shell molecules is reduced from 8.4 kcal/mol with MNDO to 6.8 kcal/mol with NO-MNDO. In addition, the performance for conformational equilibria and torsional barriers is significantly improved with NO-MNDO, presumably owing to the improved description of closed-shell interactions. For molecular geometries, the usual training and test sets have been expanded through use of MP2/6-31G(d) results for consistent comparisons. The performance of NO-MNDO for bond lengths, bond angles, and dihedral angles remains good with MAEs of 0.017 Å, 2.5°, and 4.5°. Additionally, NO-MNDO corrects severe errors by MNDO for R(•) + H-R' hydrogen-atom transfers, while testing for activation barriers for nine pericyclic reactions reveals only modest improvement.

H-C-SiH3: direct generation and spectroscopic identification of ethylidene's cousin.

Ground-state triplet silaethylidene, generated directly by the reaction of 3P carbon atoms with silane under matrix isolation conditions in solid Ar (10-12 K), has been thoroughly characterized by the EPR and IR spectra of both the parent and perdeuterated isotopologs. A theoretical anharmonic vibrational analysis based on a CCSD(T)/cc-pVTZ complete quartic force field gave remarkable agreement with the experimental IR fundamentals, generally within 10 cm-1 and without any empirical scaling of the ab initio frequencies. Silaethylidene exhibits a Cs minimum with a H-C-Si angle near 153 degrees , but the barrier to H-C-Si linearity (C3v symmetry) is only 0.24 kcal mol-1. This minuscule barrier can be surmounted by zero-point vibrations, as evident from the EPR data. The triplet stabilizing effect of the electropositive SiH3 group amounts to about 15 kcal mol-1.

On the vertical excitation energy of cyclopentadiene.

The vertical excitation energy for the lowest valence pi-->pi(*) transition of cyclopentadiene is investigated. Using a combination of high-level theoretical methods and spectroscopic simulations, the vertical separation at the ground state geometry is estimated to be 5.43+/-0.05 eV. This value is intermediate between those calculated with coupled-cluster and multireference perturbation theory methods and is about 0.13 eV higher than the observed maximum in the absorption profile.

Binding energies of small lithium clusters (Li(n)) and hydrogenated lithium clusters (Li(n)H).

Large coupled cluster computations utilizing the Dunning weighted correlation-consistent polarized core-valence (cc-pwCVXZ) hierarchy of basis sets have been conducted, resulting in a panoply of internally consistent geometries and atomization energies for small Li(n) and Li(n)H (n=1-4) clusters. In contrast to previous ab initio results, we predict a monotonic increase in atomization energies per atom with increasing cluster size for lithium clusters, in accordance with the historical Knudsen-effusion measurements of Wu. For hydrogenated lithium clusters, our results support previous theoretical work concerning the relatively low atomization energy per atom for Li(2)H compared to LiH and Li(3)H. The CCSD(T)/cc-pwCVQZ atomization energies for LiH, Li(2)H, Li(3)H, and the most stable isomer of Li(4)H, including zero-point energy corrections, are 55.7, 79.6, 113.0, and 130.6 kcal/mol, respectively. The latter results are not consistent with the most recent experiments of Wu.

Triplet H-C-SiHCl(2): combined matrix-IR and CCSD(T) identification, and the role of the open-shell singlet state.

Triplet carbene H-C-SiHCl(2) (1), prepared by a photochemical [1,2]H-shift from 1,1-dichlorosilaethylene, was identified by matching experimental and [CCSD(T)/cc-pVTZ] infrared absorptions. Parts of the potential energy surface were explored utilizing CCSD(T)/cc-pVTZ and B3LYP/6-311+G computations. DFT reproduces the experimental features and CCSD(T) computations for the triplet surface but fails in the description of the open-shell singlet state of 1. We emphasize the notion of electropositive heterosubstitution for the generation of persistent ground-state triplet carbenes.

Delocalizations in sigma-radical cations: the intriguing structures of ionized [n]rotanes.

Highly symmetric aliphatic hydrocarbons such as D(4h)-[4]rotane do not necessarily have degenerate HOMOs. According to our predictions based on high-level computations, its radical cation should display a highly delocalized D(4h)-symmetric structure, in contrast to its Jahn-Teller distorted cousin, the radical cation of [3]rotane, which exists in two distonic localized forms with C(2v) and C(s) symmetry.