Second-Order Dispersion Energy Based on Multireference Description of Monomers

Abstract

We propose a method for calculating a second-order dispersion energy for weakly interacting multireference systems in arbitrary electronic states. It is based on response properties obtained from extended random phase approximation equations. The introduced formalism is general and requires only one- and two-particle reduced density matrices of monomers. We combine the new method with either generalized valence bond perfect pairing (GVB) or complete active space (CAS) self-consistent field description of the interacting systems. In addition to a general scheme, three approximations, leading to significant reduction of the computational cost, are developed by exploiting Dyall partitioning of the monomer Hamiltonians. For model multireference systems (H2···H2 and Be···Be) the method is accurate, unlike its single-reference-based counterpart. Neither GVB nor CAS description of single-reference monomers improves the dispersion energy with respect to the Hartree–Fock-based results.

Recent developments in symmetry‐adapted perturbation theory

“…Equation ), was approximated by a weighted average of the asymptotic dipole–dipole terms corresponding to pairs of individual determinants. More recently, Hapka et al proposed a multireference approach to dispersion energy based on the extended RPA (ERPA) linear response formalism that can utilize monomer one‐ and two‐particle reduced density matrices (RDMs) from any multireference approach (in Reference , complete active space self‐consistent field [CASSCF] and the generalized VB theory were used for this purpose). The ERPA treatment approximates the full CASSCF linear response by freezing the CI coefficients of different determinants in the CASSCF wavefunction; only the orbital response is taken into account including active‐active orbital rotations.…”

“…The ERPA treatment approximates the full CASSCF linear response by freezing the CI coefficients of different determinants in the CASSCF wavefunction; only the orbital response is taken into account including active‐active orbital rotations. While the ERPA‐based dispersion energy was, predictably, unsuccessful at accounting for intramonomer dynamical correlation, the results for simple multireference systems such as H 2 –stretched H 2 and Be–Be were highly encouraging . It appears that ERPA provides a viable route to developing a complete multireference analog of SAPT0.…”

Second-Order Exchange-Dispersion Energy Based on a Multireference Description of Monomers

“…Finally, we observe that the errors in exchange-dispersion energies reported in Table 2 and Figure 3 are overall higher than those of the second-order dispersion energy presented in Ref. 34. For instance, the mean absolute error for the TK21 data set in dispersion calculated using CASSCF description of monomers amounts to 7.0% (cf.…”

“…Similar behavior was observed for the E (2) disp energy in our previous work. 34 It should be emphasized, that the true target of the ERPA-based SAPT variant, are many-electron systems of a multirefence character for which neither Hartree-Fock nor other single-reference SAPT formulations are reliable.…”

Assessment of SAPT(DFT) with meta-GGA functionals

“…Korona employed CCSD density matrices and response functions in the formulation of SAPT(CCSD) which also bypasses double-perturbation theory [9]. Finally, Hapka et al investigated the extent to which the complete active space monomers' wave functions recover the intrasystem correlation effects on the dispersion energy [10,11].…”