Derek Walter

Local correlation in the virtual space in multireference singles and doubles configuration interaction

We describe a multireference configuration interaction method that takes advantage of local correlation methods in both the internal (originally occupied) and external (originally unoccupied or virtual) orbital spaces. In the internal space, implementation of local correlation is trivial and involves neglecting configurations having simultaneous excitations out of widely separated orbitals. In the external space, the method involves restricting the space of allowed correlating orbitals to those localized near the hole orbitals. Of course, this necessitates the use of localized virtual orbitals which in turn requires one to sacrifice the orthogonality of the virtual space. This complicates the formalism substantially, and we discuss the necessary changes to the traditional expressions in detail. The scaling of the method with system size, basis set size, and the average number of allowed virtual orbitals is explored. An examination of systems having up to 8 heavy atoms reveals that the computational costs ...

Size extensive modification of local multireference configuration interaction

We recently developed a reduced scaling multireference configuration interaction (MRCI) method based on local correlation in the internal (occupied) and external (virtual) orbital spaces. This technique can be used, e.g., to predict bond dissociation energies in large molecules with reasonable accuracy. However, the inherent lack of size extensivity of truncated CI is a disadvantage that in principle worsens as the system size grows. Here we implement an a priori size-extensive modification of local MRCI known as the averaged coupled pair functional (ACPF) method. We demonstrate that local MR-ACPF recovers more correlation energy than local MRCI, in keeping with trends observed previously for nonlocal ACPF. We test the size extensivity of local ACPF on noninteracting He atoms and a series of hydrocarbons. Basis set and core correlation effects are explored, as well as bond breaking in a variety of organic molecules. The local MR-ACPF method proves to be a useful tool for investigating large molecules and represents a further improvement in predictive accuracy over local MRCI.

Local weak-pairs pseudospectral multireference configuration interaction

We present a new reduced scaling multireference singles and doubles configuration interaction (MRSDCI) algorithm based upon the combination of local correlation and pseudospectral methods. Taking advantage of the locality of the Coulomb potential, the weak-pairs approximation of Saebo/ and Pulay is employed to eliminate configurations having simultaneous excitations out of pairs of distant, weakly interacting orbitals. In conjunction with this, the pseudospectral approximation is used to break down the most time-consuming two-electron integrals into a product of intermediate quantities depending on no more than two orbital indices. The resulting intermediate quantities are then used directly in the CI equations to substantially reduce the number of floating point operations required for diagonalization of the Hamiltonian. Additionally, our CI algorithm is based upon the symmetric group graphical approach CI (SGGA-CI) of Duch and Karwowski. For the purpose of developing reduced scaling CI algorithms, this approach has some important advantages. The most important of these advantages are the on-the-fly calculation of integral coupling coefficients and the separation of the spin and spatial parts of the wave function, which simplifies implementation of local correlation approximations. We apply the method to determine a series of binding energies in hydrocarbons and show that the approximate method predicts binding energies that are within a few kcal/mol of those predicted by the analytic nonlocal method. For large molecules, the local pseudospectral method was shown to be over 7 times as fast as the analytic nonlocal method. We also carry out a systematic study on the performance of different basis sets in the weak-pairs method. It was determined that triple-ζ basis sets were capable of recovering only 99.0% of the correlation energy, whereas double-ζ basis sets recovered 99.9% of the correlation energy.

Multi-reference weak pairs local configuration interaction: efficient calculations of bond breaking

We present a new local multi-reference singles and doubles configuration interaction (MRSDCI) algorithm. The method presented here eliminates configurations if they involve simultaneous excitations out of widely separated internal orbitals and is therefore based on the weak pairs approximation of Saebo and Pulay. Although the resulting truncated CI expansions have only about 50% as many CSFs as the non-local MRSDCI, we show that they can recover over 99% of the correlation energy. Additionally, we show for the first time that they can accurately describe bond dissociation.

Determination of weak Fe+–L bond energies (L = Ar, Kr, Xe, N2, and co2) by ligand exchange reactions and collision-induced dissociation

Abstract Guided-ion beam mass spectrometry is used to study the ligand exchange and collision induced dissociation reactions of Fe+(N2) with Ar, Kr, Xe, CO2, and CD4 and Fe+(CO2) with Kr, Xe, and N2 as a function of kinetic energy. Analysis of the energy dependent cross sections provides threshold energies for both types of reactions. These thresholds can be converted to the following 0 K bond dissociation energies: D0(Fe+–Ar) = 0.11 ± 0.08 eV, D0(Fe+–Kr) = 0.31 ± 0.07 eV, D0(Fe+–Xe) = 0.44 ± 0.06 eV, D0(Fe+–N2) = 0.55 ± 0.04 eV, and D0(Fe+–CO2) = 0.62 ± 0.04 eV. Our results are compared with experimental and theoretical values found in the literature. These comparisons suggest that Fe+(CO2) has a linear structure in agreement with theoretical calculations.

Alkali ion carbonyls: Sequential bond energies of Li+(CO)x (x = 1-3), Na+(CO)x (x = 1, 2) and K+(CO)

The sequential bond dissociation energies for Li+(CO)x (x = 1−3), Na+(CO)x (x = 1,2), and K+(CO) are determined by examining the collision-induced dissociation reactions with argon in a guided ion beam mass spectrometer. Analysis of the kinetic energy dependent cross sections yield values for the (CO)x−1M+CO bond dissociation energies (BDEs) of 0.57 ± 0.13, 0.37 ± 0.04 and 0.36 ± 0.04 eV for M  Li (x = 1–3, respectively), 0.33 ± 0.08 and 0.25 ± 0.03 eV for M  Na (x = 1 and 2, respectively), and 0.19 ± 0.05 eV for K+CO. In addition, the M+Ar BDEs are determined by examining the ligand exchange reaction of M+(CO) with Ar and are found to be 0.34 ± 0.14, 0.16 ± 0.09 and 0.14 ± 0.07 eV for M  Li, Na and K, respectively. The trends in BDEs can be explained readily in terms of electrostatic bonding interactions. A comparison of the alkali metal ion—CO BDEs with those of transition metal ion carbonyls reveals the contributions of s-dσ hybridization and d-π∗ back-donation in the chemical bonding of the latter systems.