A comparative assessment of the perturbative and renormalized coupled cluster theories with a noniterative treatment of triple excitations for thermochemical kinetics, including a study of basis set and core correlation effects

Abstract

Abstract. The CCSD, CCSD(T), and CR-CC(2,3) coupled cluster methods, combined with five triple-zeta basis sets, namely MG3S, aug-cc-pVTZ, aug-cc-pV(T+d)Z, aug-cc-pCVTZ, and aug-cc-pCV(T+d)Z, are tested against the DBH24 database of diverse reaction barrier heights. The calculations confirm that the inclusion of connected triple excitations is essential to achieving high accuracy for thermochemical kinetics. They show that various non-iterative ways of incorporating connected triple excitations in coupled cluster theory, including the CCSD(T) approach, the full CR-CC(2,3) method, and approximate variants of CR-CC(2,3) similar to the triples corrections of the CCSD(2) approaches, are all about equally accurate for describing the effects of connected triply excited clusters in studies of activation barriers. The effect of freezing core electrons on the results of the CCSD, CCSD(T), and CR-CC(2,3) calculations for barrier heights is also examined. It is demonstrated that to include core correlation most reliably, a basis set including functions that correlate the core and that can treat core-valence correlation is required. On the other hand, the frozen-core approximation using valence-optimized basis sets that lead to relatively small computational costs of CCSD(T) and CR-CC(2,3) calculations can achieve almost as high accuracy as the analogous fully correlated calculations.

Block Correlated Coupled Cluster Theory With A Complete Active-Space Self-Consistent-Field Reference Function: The General Formalism And Applications

“…However, it has been found that these methods can not provide balanced descriptions for different regions of potential energy surfaces, due to the spin contamination of the UHF reference function [13,14]. Besides these two approaches, other effective approaches have also been proposed, which include: the reduced multireference CCSD [15][16][17][18][19][20][21][22][23][24], the spin-flip method [25][26][27][28][29][30][31][32], the orbital-optimized CC approach [33][34][35][36][37][38][39][40], the completely renormalize CC approaches [41][42][43][44][45][46][47][48][49], active-space CC approaches [50][51][52][53][54][55][56][57][58][59][60]<...>…”

Rate coefficients of the CF3CHFCF3 + H → CF3CFCF3 + H2 reaction at different temperatures calculated by transition state theory with ab initio and DFT reaction paths

“…In addition, as a result of rapid advances in computational technology in the last decade, computational chemists can now carry out quantum chemical and transition state theory (TST) calculations at a higher level than previously possible. Also, in relation to reaction kinetics, there have been considerable efforts in recent research in ab initio [7–13] and density functional theory (DFT)[14–19] methods aiming to calculate reliable reaction barriers, and in TST to calculate accurate reaction rate coefficients. [20–22] These efforts have led to recent calculations of highly accurate reaction rate coefficients by employing ab initio and/or DFT reaction paths or energy surfaces, and TST of various degrees of sophistication.…”

An Efficient and Accurate Formalism for the Treatment of Large Amplitude Intramolecular Motion

“…The CR-CC(2,3) method is an improvement over the CCSD(T) approach to overcome its deficiencies in describing systems involving biradical character. 61 Geometry optimizations are carried out at the UB3LYP 51 /6-31G(d) 52 level of theory. All the electronic calculations have been performed using the GAMESS code.…”