The accuracy of ab initio molecular geometries for systems containing second-row atoms

Abstract

The performance of the standard hierarchy of ab initio models--that is, Hartree-Fock theory, second-order Moller-Plesset theory, coupled-cluster singles-and-doubles theory, and coupled-cluster singles-doubles-approximate-triples theory--in combination with correlation-consistent basis sets is investigated for equilibrium geometries of molecules containing second-row elements. From an analysis on a collection of 31 molecules (yielding statistical samples of 41 bond distances and 13 bond angles), the statistical errors (mean deviation, mean absolute deviation, standard deviation, and maximum absolute deviation) are established at each level of theory. The importance of core correlation is examined by comparing calculations in the frozen-core approximation with calculations where all electrons are correlated.

The Case of the Weak N−X Bond: Ab Initio, Semi-Experimental, and Experimental Equilibrium Structures of XNO (X = H, F, Cl, OH) and FNO2

“…In particular, Lee 66 used the CCSD(T) level of theory but, as for HONO, with the TZ2P basis set, which is slightly too small. Quite recently, Coriani et al 15 recalculated the structure of ClNO at the CVQZ CCSD(T) level, all electrons being correlated. They noticed a disagreement with the experimental structure for the r e (N-Cl) bond length and suggested to revise the experimental determination.…”

“…This is not completely surprising because these molecules contain several atoms with large electronegativities and HONO, for example, is isoelectronic with O 3 , a notoriously difficult molecule to describe at low levels of electronic structure theory. Nevertheless, it has been argued 15 that the coupled-cluster (CC) method with single and double excitations (CCSD) 16 augmented by a perturbational estimate of the effects of connected triple excitations [CCSD(T)] 17 gives accurate results for these molecules. However, when the ab initio structures are compared to dependable experimental ones, as done in this study, a rather poor agreement is found for the length of the N-X bond though not for the other structural parameters.…”

Avoiding pitfalls of a theoretical approach: the harmonic oscillator measure of aromaticity index from quantum chemistry calculations

“…Judging from the studies concerning the accuracy of ab initio methods for prediction of molecular equilibrium structures [19][20][21]23], the best method that is feasible for medium size molecules (up to 6-9 heavy atoms, depending on the symmetry of the system) appears to be the coupledclusters singles and doubles, with perturbative inclusion of triples-CCSD(T), especially when combined with the cc-pVTZ or, preferably, with the cc-pVQZ basis sets. The mean error of this computational scheme ( " D), determined for a set of molecules containing first and second row atoms, is less than 1 pm, with the maximum absolute deviation D max j j=1.511 pm.…”

“…The whole paper will be divided into two main sections. First, we will present the benchmark HOMA values for the selected unsaturated hydrocarbons obtained by means of the CCSD(T) computational scheme, which was reported to provide accuracy comparable to that of the best experiments [19][20][21]. Subsequently, the benchmark will be used to test the performance of the DFT method, with a choice of 25 exchange-correlation functionals destined to various fields of chemistry, and two basis sets of different sizes.…”

Ab initio calculations on SCl2 and low-lying cationic states of SCl2+: Franck-Condon simulation of the UV photoelectron spectrum of SCl2

“…[11][12][13][14] However, the only ab initio investigation on SCl 2 + is the modified coupled-pair functional ͑MCPF͒ calculations on the S 2p core-hole states of SCl 2 + . 15 There is no calculation available on any low-lying, valence cationic state of SCl 2 + to our knowledge.…”