Abstract
A multireference perturbation method is formulated, that uses an
optimized partitioning. The zeroth-order energies are chosen in a way
that guarantees vanishing the first neglected term in the
perturbational ansatz for the wave function, Ψ(n) =
0. This procedure yields a family of zeroth-order Hamiltonians that
allows for systematic control of errors arising from truncating the
perturbative expansion of the wave function. The second-order version
of the proposed method, denoted as MROPT(2), is shown to be (almost)
size-consistent. The slight extensivity violation is shown
numerically. The total energies obtained with MROPT(2) are similar to
these obtained using the multireference configuration interaction
method with Davidson-type corrections. We discuss connections of the
MROPT(2) method to related approaches, the optimized partitioning
introduced by Szabados and Surjan and the linearized multireference
coupled-cluster method. The MROPT(2) method requires using
state-optimized orbitals; we show on example of N2 that
using Hartree–Fock orbitals for some excited states may lead to
nonphysical results.