Numerically oriented static response approach based on state-specific multi-reference coupled electron-pair approximation (SS-MRCEPA) like methods

In this paper, we have exploited the recently developed state-specific multi-reference coupled electron-pair approximation (SS-MRCEPA) like methods for computing the electrostatic response properties. The SS-MRCEPA methods are formulated on complete active space reference functions and the required energies are computed via the diagonalization of an effective operator within this space. The SS-MRCEPA theories are size-consistent and size-extensive. They are very efficient for treating quasidegeneracy of varying extent and for bypassing the intruder problem. The efficacy of the methods is illustrated via the computation of the static dipole moment and polarizability of the ground state and the corresponding first excited state of the trapezoidal H-4 model (H4) as well as the lowest two singlet states of the CH2 system using-perturbed orbitals generated via the finite-field strategy, a numerically oriented static response approach.