No abstract

In recent literature, some authors claim to have successfully applied density functional theory (DFT) methods to the attractive interaction between rare-gas atoms. In this note, we make a critical survey of these works and come to the conclusion that, in contrast to the claims made, state-of-the-art DFT methods are incapable of accounting for dispersion effects in a quantitative way.

The performance of a new crystal packing procedure for the ab initio prediction of possible molecular crystal structures is presented. The method is based upon only molecular information, i.e., no unit cell parameters are assumed to be known. The search for the global crystal energy minimum and all local minima inside an energy window is derived from Monte Car10 simulated annealing methods and has been applied to various organic molecules containing heteroatoms and polar groups. A systematic evaluation of the search method and of the quality of the potential energy function has been established. It is demonstrated that the packing of general organic molecules is possible even with standard force fields like CHARMM provided that the charges defining the electrostatic interactions are based upon physical models rather than transferable empirical parameters. Concepts of crystal packing that were based till now upon assumptions and speculations could be proved or disproved by solving directly the extended global optimization problem related to crystal packing. Crystal structures of molecules as complex as those treated in this article have not been, till now, predicted by a computational approach. In one case, a disagreement between the predicted and experimental structure was evident and, based upon the computations, we suspect that the published structure is the wrong one.

No abstract

ABSTRACT:The averaged coupled-pair functional (ACPF), as found in R. J. Gdanitz and R. Ahlrichs, Chem. Phys. Lett. 143 (1988), is probably the most successful method to approximate full configuration interaction (CI) on the multireference (MR) level of theory. However, ACPF has a tendency to slightly overestimate the effect of higher than double substitutions, which, when the zeroth-order wavefunction is of poor quality, may deteriorate the accuracy or even create instabilities. Since the properties of the ACPF and similar methods have apparently not always been correctly described in the literature, we repeat the derivation of this method in some detail. We analyze the connection between the (original) ACPF and the similar averaged quadratic coupled cluster (AQCC) method (which may be regarded as a damped ACPF), on one hand, and the different versions (0-3) of the coupled electron-pair approximation (CEPA), on the other hand. We find that ACPF and AQCC may be regarded as CEPA-1, respectively, CEPA-3, where the shifts of the Hamiltonian are substituted by a single averaged shift. As CEPA-3 considerably underestimates correlation effects, AQCC shows the same behavior. However, when the zeroth-order wavefunction is of poor quality, AQCC may be more stable and thus more accurate than ACPF. By analyzing the role of the single substitutions, we find that ACPF may especially overestimate their contribution to unlinked clusters like 1 2T 2 1 . We therefore propose a new version, called ACPF-2, where (in contrast to AQCC) only the renormalization factor, g, that corresponds to the singles, is damped; i.e., we have (4/N)[1 − 1/(2N − 2)]. In the limit of a large number of electrons, N, this factor becomes two times as large as in the (original) ACPF, where 2/N is used. In order to test the new ACPF-2 method, we perform numerous comparisons with full CI, as well as calculations including terms that are linear in the interelectronic distances, r ij . We find that in difficult cases, ACPF-2 is of similar stability as is AQCC and is thus considerably more accurate GDANITZ than the original ACPF. In contrast to AQCC, however, ACPF-2 achieves this stability without sacrificing the high accuracy which is obtained by ACPF in well-behaved cases.