A linear scaling multireference singles and doubles configuration interaction (MRSDCI) method has been developed. By using localized bases to span the occupied and virtual subspace, local truncation schemes can be applied in tandem with integral screening to reduce the various bottlenecks in a MRSDCI calculation. Among these, the evaluation of electron repulsion integrals and their subsequent transformation, together with the diagonalization of the large CI Hamiltonian matrix, correspond to the most computationally intensive steps in a MRSDCI calculation. We show that linear scaling is possible within each step. The scaling of the method with system size is explored with a system of linear alkane chains and we proceed to demonstrate this method can produce smooth potential energy surfaces via calculating the dissociation of trans-6-dodecene (C(12)H(24)) along the central C[Double Bond]C bond.

A local multireference singles and doubles configuration interaction method in which Cholesky vectors are used in place of conventional two-electron integrals has been developed (CD-LMRSDCI). To reduce the overall cost associated with our linear scaling LMRSDCI method presented earlier [T. S. Chwee et al., J. Chem. Phys. 128, 224106 (2008)], we adopt a two-pronged approach. First, localized orthogonal virtual orbitals, introduced by Subotnik et al. [J. Chem. Phys. 123, 114108 (2005)], are substituted for nonorthogonal projected atomic orbitals. This obviates the need for contraction with overlap matrices and simplifies our working formalism. In addition, we restructure the rate-limiting step of our LMRSDCI algorithm to be driven by the search for two-electron integrals instead of configuration state functions. The shift necessitates a flexible way of processing the four-indexed two-electron integrals, which is facilitated by use of two-indexed Cholesky vectors. Our restructured LMRSDCI method is an order of magnitude faster and has greatly reduced storage requirements so that we are able to apply it to molecules containing up to 50 heavy atoms. However, generation of the Cholesky vectors and their subsequent transformation to the molecular orbital (MO) basis is not linear scaling. Together with assembling the MO integrals from the Cholesky vectors, these now constitute the rate-limiting steps in our method.

The adsorption energies and changes in surface work functions for benzene on unreconstructed Cu(111), Ag (111), and Au (111) at low coverages have been studied within the framework of dispersion corrected Kohn-Sham density functional theory. Corrections to account for long range dispersive effects between the adsorbate and metal substrate were incorporated via the exchange-hole dipole moment method of Becke and Johnson [J. Chem. Phys. 123, 154101 (2005)]. We show that the dispersion corrected calculations yield significantly improved adsorption energies and work function shifts that are in good agreement with experimental values.

Topological insulators (TIs) represent an exciting new class of materials with potential applications in spintronics and quantum computing. In this work, we present a theoretical study on a new family of two dimensional (2D) nanomaterials based on the coordination of shape persistent organic ligands (SPOLs) to heavy transition metal ions such as Pd(2+) and Pt(2+). These 2D structures may be readily fabricated and are expected to be stable under normal atmospheric conditions. From first principles calculations and tight-binding model simulations carried out to characterize the bulk band structures, edge states, spin Chern numbers, and the Z2 topological invariants, we were able to identify candidates with non-trivial topological properties that may serve as topological insulators in real world applications.