Geometry optimizations with the coupled-cluster model CC2 using the resolution-of-the-identity approximation

Hättig, Christof

doi:doi:10.1063/1.1564061

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Monte Carlo simulations using sampling from an approximate potential

A simulation algorithm is proposed in which the potential energy function used in a Monte Carlo simulation is replaced with one that is less expensive to evaluate, coupled with a correction step based on the difference between the two potentials. This can result in a substantial reduction in computational cost. A formal derivation of the appropriate sampling criteria is given, as well as estimates of the possible improvements in code performance. The method is demonstrated on the Lennard-Jones f...

A novel path sampling method for the calculation of rate constants

We derive a novel efficient scheme to measure the rate constant of transitions between stable states separated by high free energy barriers in a complex environment within the framework of transition path sampling. The method is based on directly and simultaneously measuring the fluxes through many phase space interfaces and increases the efficiency with at least a factor of 2 with respect to existing transition path sampling rate constant algorithms. The new algorithm is illustrated on the isom...

An implementation of the gradient for the second-order coupled-cluster singles-and-doubles model CC2 is reported, which employs the resolution-of-the-identity (RI) approximation for electron repulsion integrals. The performance of the CC2 model for ground state equilibrium geometries and harmonic frequencies is investigated and compared with experiment and other ab initio methods. It is found that CC2 equilibrium geometries have a similar accuracy to those calculated with second-order Møller–Plesset perturbation theory (MP2), but the bond lengths are larger. In particular, double and triple bonds and bonds in electron-rich compounds are elongated by 0.5–1.5 pm. Thereby CC2 slightly outperforms MP2 for single bonds, in particular in electron-rich compounds, but for strong double and triple bonds CC2 is somewhat inferior to MP2. The results for harmonic frequencies go in parallel with the results for equilibrium structures. The error introduced by the RI approximation is found to be negligible compared to the remaining one-electron basis set error, if optimized auxiliary basis sets are used. Typically, the RI error in bond lengths is of the order of 10⁻³ pm and the error in angles 10⁻³–10⁻² deg. Applications are reported for the geometry of trans-azobenzene and for the geometry and harmonic frequencies of cis,trans-1,4-difluorobutadiene. © 2003 American Institute of Physics.