title: SF-[2]R12: a spin-adapted explicitly correlated method applicable to arbitrary electronic states.
abstract: The [2](R12) method [M. Torheyden and E. F. Valeev, J. Chem. Phys. 131, 171103 (2009)] is an explicitly correlated perturbative correction that can greatly reduce the basis set error of an arbitrary electronic structure method for which the two-electron density matrix is available. Here we present a spin-adapted variant (denoted as SF-[2](R12)) that is formulated completely in terms of spin-free quantities. A spin-free cumulant decomposition and multi-reference generalized Brillouin condition are used to avoid three-particle reduced density matrix completely. The computational complexity of SF-[2](R12) is proportional to the sixth power of the system size and is comparable to the cost of the single-reference MP2-R12 method. The SF-[2](R12) method is shown to decrease greatly the basis set error of multi-configurational wave functions.
authors.affiliation: Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, USA. liguokong@gmail.com
authors.forename: Liguo
authors.initials: L
authors.lastname: Kong
authors.forename: Edward F
authors.initials: EF
authors.lastname: Valeev