title: Investigating Multireference Character and Correlation in Quantum Chemistry.
abstract: We review a range of multireference diagnostics for quantum chemistry and discuss them in terms of choices of the molecular orbitals. We show how an approach1 of P.-O. Löwdin can also be viewed as quantifying the electron correlation via the spatial entanglement relative to a single determinant. We consider three example systems from quantum chemistry that exhibit three different combinations of multireference character and correlation: not strongly multireference and not strongly correlated, strongly multireference but not strongly correlated, and strongly multireference together with strong correlation. We find that a multireference measure (MR) does not change substantially with the cutoff used for a Monte Carlo configuration interaction calculation and investigate the effect of using natural orbitals. We see that a coupled-cluster singles and doubles diagnostic and a density-functional theory diagnostic give a correct general prediction of the multireference character for these systems. We also look at the issue of multireference character for a collection of noninteracting hydrogen molecules and the effect of basis size on the multireference character of a stretched hydrogen molecule.
authors.affiliation: Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University , Edinburgh, EH14 4AS, United Kingdom.
authors.forename: J P
authors.initials: JP
authors.lastname: Coe
authors.affiliation: Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University , Edinburgh, EH14 4AS, United Kingdom.
authors.forename: M J
authors.initials: MJ
authors.lastname: Paterson