Variational two-electron reduced density matrix theory for many-electron atoms and molecules: Implementation of the spin- and symmetry-adapted T₂ condition through first-order semidefinite programming

The energy and properties of a many-electron atom or molecule may be directly computed from a variational optimization of a two-electron reduced density matrix (2RDM) that is constrained to represent many-electron quantum systems. In this paper we implement a variational 2RDM method with a representability constraint, known as the T₂ condition. The optimization of the 2RDM is performed with a first-order algorithm for semidefinite programming [ D. A. Mazziotti Phys. Rev. Lett. 93 213001 (2004)] which, because of its lower computational cost in comparison to second-order methods, allows the treatment of larger basis sets. We also derive and implement a spin- and symmetry-adapted formulation of the T₂ condition that significantly decreases the size of the largest block in the T₂ matrix. The T₂ condition, originally derived by Erdahl Int. J. Quantum Chem. 13 697 (1978), was recently applied via a second-order algorithm to atoms and molecules [ Z. Zhao et al. J. Chem. Phys. 120 2095 (2004)]. While these calculations were restricted to molecules at equilibrium geometries in minimal basis sets, we apply the 2RDM method with the T₂ condition to compute the electronic energies of molecules in both minimal and nonminimal basis sets at equilibrium as well as nonequilibrium geometries. Accurate potential energies curves are produced for BH, HF, and N₂. Results are compared with the 2RDM method without the T₂ condition as well as several wave-function methods.