Multireference Second-Order Perturbation Theories Based on Iterative Configuration Interaction with Selection
Year: 2026
Type: preprint
Abstract: Under the assumption that the static correlation can well be described by selected configuration interaction, to the extent that the residual dynamic correlation can be accounted for accurately by a second-order perturbation theory (PT2), a family of multireference PT2s (iCI-MRPT2) based on the iterative configuration interaction with selection are proposed and compared. The first step is to partition a large complete active space (CAS) $P$ into the direct sum of a primary ($P_m$) and a secondary ($P_s$) subspace via a parameter $C_{\mathrm{min}}$. The former serves as the actual reference space, whereas the latter is treated via Epstein-Nesbet PT2. An MRPT2 is then invoked to account for the dynamic correction of the $Q=1-P$ space to the $P_m$ space. Different MRPT2s are distinguished by the underlying zeroth-order Hamiltonians. To circumvent the prohibitive computational cost and memory footprint associated with high-order reduced density matrices in the fully internally contracted scheme and the exponential growth of external functions (XF) in the uncontracted scheme, a partial contraction strategy is employed for constructing the XFs: those involving one or two inactive (core or virtual) orbitals are kept uncontracted, whereas those involving three or four inactive orbitals are internally contracted. They span the uncontracted ($Q_u$) and contracted ($Q_c$) portions of $Q$, respectively. The former is further split into two subspaces $Q_u^c$ and $Q_u^u$ via a parameter $Q_{\mathrm{min}}$, which are to be treated in coupled and uncoupled manners, respectively. Extrapolations to zero $Q_{\mathrm{min}}$ and $C_{\mathrm{min}}$ can be performed, so as to approach CAS-based MRPT2. Calculations with active spaces as large as CAS(56e,58o) can now be performed. The size-consistency of the iCI-MRPT2s is also analyzed in detail.
Source: arXiv (Cornell University)
Authors: Zhang, Ning, Liu, Wenjian
Institution: Shandong University
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: G7794904308, G8957963440
OpenAlex ID: W7224116242