Analytical energy gradients for local second-order Mo/ller–Plesset perturbation theory

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302

An efficient local coupled cluster method with single and double excitation operators and perturbative treatment of triple excitations [DF-LCCSD(T)] is described. All required two-electron integrals are evaluated using density fitting approximations. These have a negligible effect on the accuracy but reduce the computational effort by 1-2 orders of magnitude, as compared to standard integral-direct methods. Excitations are restricted to local subsets of non-orthogonal virtual orbitals (domain approximation). Depending on distance criteria, the correlated electron pairs are classified into strong, close, weak, and very distant pairs. Only strong pairs, which typically account for more than 90% of the correlation energy, are optimized in the LCCSD treatment. The remaining close and weak pairs are approximated by LMP2 (local second-order Møller-Plesset perturbation theory); very distant pairs are neglected. It is demonstrated that the accuracy of this scheme can be significantly improved by including the close pair LMP2 amplitudes in the LCCSD equations, as well as in the perturbative treatment of the triples excitations. Using this ansatz for the wavefunction, the evaluation and transformation of the two-electron integrals scale cubically with molecular size. If local density fitting approximations are activated, this is reduced to linear scaling. The LCCSD iterations scale quadratically, but linear scaling can be achieved by neglecting some terms involving contractions of single excitations. The accuracy and efficiency of the method is systematically tested using various approximations, and calculations for molecules with up to 90 atoms and 2636 basis functions are presented.

Section: Choice Of Orbital Domainsmentioning

confidence: 99%

An efficient local coupled cluster method for accurate thermochemistry of large systems

Werner

Schütz

2011

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“…All quantum chemistry calculations were performed with the MOLPRO software. 36 For each dye in Scheme 1, we obtained a ground state minimum geometry using an MP2 37 model with a cc-pvdz basis set 35 (MP2/cc-pvdz). The Cartesian coordinates of these geometries are available in the supplementary material.…”

Section: A Computationsmentioning

confidence: 99%

A three-state effective Hamiltonian for symmetric cationic diarylmethanes

Olsen

McKenzie

2012

Comparison of density functionals for energy and structural differences between the high-[ 5 T 2g :(t 2g ) 4 (e g ) 2 ] and low-[ 1 A 1g :(t 2g ) 6 (e g ) 0 ] spin states of iron (II) We analyze the low-energy electronic structure of a series of symmetric cationic diarylmethanes, which are bridge-substituted derivatives of Michler's Hydrol Blue. We use a four-electron, threeorbital complete active space self-consistent field and multi-state multi-reference perturbation theory model to calculate a three-state diabatic effective Hamiltonian for each dye in the series. We exploit an isolobal analogy between the active spaces of the self-consistent field solutions for each dye to represent the electronic structure in a set of analogous diabatic states. The diabatic states can be identified with the bonding structures in classical resonance-theoretic models of cyanine dyes. We identify diabatic states with opposing charge and bond-order localization, analogous to the classical resonance structures, and a third state with charge on the bridge. While the left-and right-charged structures are similar for all dyes, the structure of the bridge-charged diabatic state, and the Hamiltonian matrix elements connected to it, change significantly across the series. The change is correlated with an inversion of the sign of the charge carrier on the bridge, which changes from an electron pair to a hole as the series is traversed.

“…In this way, an N-scaling behaviour may be obtained. Without being exhaustive, a lot of developments may be mentioned, concerning self-consistent field (SCF), second-order Moller-Plesset (MP2), coupled-cluster single double (triple) (CCSD(T)), or MP2 gradient, [13][14][15][16][17][18] and others.…”

Section: Introductionmentioning

confidence: 99%

Multi-scale multireference configuration interaction calculations for large systems using localized orbitals: Partition in zones

Chang

Calzado

Amor

et al. 2012

A new multireference configuration interaction method using localised orbitals is proposed, in which a molecular system is divided into regions of unequal importance. The advantage of dealing with local orbitals, i.e., the possibility to neglect long range interaction is enhanced. Indeed, while in the zone of the molecule where the important phenomena occur, the interaction cut off may be as small as necessary to get relevant results, in the most part of the system it can be taken rather large, so that results of good quality may be obtained at a lower cost. The method is tested on several systems. In one of them, the definition of the various regions is not based on topological considerations, but on the nature, σ or π , of the localised orbitals, which puts in evidence the generality of the approach.