Systematic truncation of the virtual space in multiconfigurationalperturbation theory

摘要

A method is suggested which allows truncation of the virtual space in Choleskydecomposition-based multiconfigurational perturbation theory (CD-CASPT2) calculations withsystematic improvability of the results. The method is based on a modified version of the frozennatural orbital (FNO) approach used in coupled cluster theory. The idea is to exploit the near-lineardependence among the eigenvectors of the virtual-virtual block of the second-order Moller-Plessetdensity matrix. It is shown that FNO-CASPT2 recovers more than 95% of the full CD-CASPT2correlation energy while requiring only a fraction of the total virtual space, especially when largeatomic orbital basis sets are in use. Tests on various properties commonly investigated withCASPT2 demonstrate the reliability of the approach and the associated reduction in computationalcost and storage demand of the calculations.