A study of molecular dissociation within the unrestricted doubly-occupied configuration interaction scheme

JAVIER GARCIA; OFELIA B. OÑA; DIEGO R. ALCOBA; ALICIA TORRE; LUIS LAIN

Torun

Congreso; QSCP 2022 - 25th International Workshop on Quantum Systems in Chemistry, Physics and Biology; 2022

Universidad Nicolaus Copernicus

The doubly-occupied configuration interaction method (DOCI) consists of limiting a configuration interaction determinantal expansion to only include doubly occupied Slater determinants.It has been shown that DOCI captures most of the static correlation, and it has been proposed as an alternative to excitation-based truncations of the full configuration interaction (full CI).Additionally, the 2-particle reduced density matrix (2RDM) can be obtained variationally for such systems with polynomial cost, therefore providing a valid alternative to multireferential SCF methods. The eigenenergies of the DOCI 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 DOCI captures most of the static correlation, it still gives a wrong description of molecules in the dissociation limit when using the optimal orbital basis.By performing independent optimizations of the alpha and beta orbitals, we show that for some simple systems such as NH$_3$ and N$_2$, the unrestricted version of DOCI (UDOCI) gives the correct description of molecular dissociation, yielding the correct energies, and local spins, at the penalty of giving an incorrect description of the global spin.pre { background: transparent }pre.western { font-family: "Liberation Mono", monospace; font-size: 10pt }pre.cjk { font-family: "Noto Sans Mono CJK SC", monospace; font-size: 10pt }pre.ctl { font-family: "Liberation Mono", monospace; font-size: 10pt }p { line-height: 115%; margin-bottom: 0.1in; background: transparent }a:link { color: #000080; so-language: zxx; text-decoration: underline }a:visited { color: #800000; so-language: zxx; text-decoration: underline }