A Study Of Molecular Dissociation Within The Unrestricted Doubly-Occupied Configuration Interaction Scheme

J. GARCIA; O.B. OÑA; D.R. ALCOBA; A. TORRE; L. LAIN

Torun

Workshop; 25th International Workshop On Quantum Systems In Chemistry, Physics And Biology (QSCP); 2022

The restricted doubly-occupied configuration interaction (RDOCI) method expresses the N-electron system wave function in terms of doubly occupied Slater determinant basis sets. It has been proved that the RDOCI method captures most of the static correlation and, consequently, it has been proposed as an alternative to excitation-based truncations of the full configuration interaction (FCI) treatment [1]. Additionally, within the doubly-occupied space, the two-particle reduced density matrix (2RDM) can be obtained variationally at polynomial cost, providing a valid alternative to multireferential SCF methods; the method has been called v2RDM-RDOCI [2,3]. The eigenenergies of the RDOCI Hamiltonian depend on the single-particle basis set employed, and therefore a minimization with respect to orbital rotations is required. It is known that even though the orbital-optimized RDOCI method provides a good description of molecules near equilibrium internuclear distances, it can fail in predicting dissociation products [1]. By performing independent optimizations of the alpha and beta spin-orbitals, we show that for some simple systems such as NH3, N2, and their isoelectronic systems CH3- , OH3+, and C2-2 , the unrestricted version of RDOCI method (UDOCI) [4,5] as well as its variational version (v2RDM-UDOCI) [5] give the correct description of molecular dissociation, yielding the correct energies and local spins, albeit with the shortcoming of providing an incorrect description of global spin.References[1] L. Byatutas et al., J. Chem. Phys 135, 044119 (2011).[2] W. Poelmans et al., J. Chem. Theory Comput. 11, 4064 (2015).[3] D. R. Alcoba et al., J. Chem. Phys. 148, 024105 (2018).[4] P. A. Limacher et al., Mol. Phys. 112, 853 (2014).[5] D. R. Alcoba et al., J. Chem. Phys. 150, 164106 (2019).