Development of Monte Carlo configuration interaction: Natural orbitals and second-order perturbation theory

Self Cite

We introduce state-averaging into the method of Monte Carlo configuration interaction (SA-MCCI) to allow the stable and efficient calculation of excited states. We show that excited potential curves for H 3 , including a crossing with the ground state, can be accurately reproduced using a small fraction of the FCI space. A recently introduced error measure for potential curves [J. P. Coe and M. J. Paterson, J. Chem. Phys., 137, 204108 (2012)] is shown to also be a fair approach when considering potential curves for multiple states. We demonstrate that potential curves for LiF using SA-MCCI agree well with the FCI results and the avoided crossing occurs correctly. The seam of conical intersections for CH 2 found by Yarkony [J. Chem. Phys., 104, 2932 (1996)] is used as a test for SA-MCCI and we compare potential curves from SA-MCCI with FCI results for this system for the first three triplet states. We then demonstrate the improvement from using SA-MCCI on the dipole of the 2 1 A 1 state of carbon monoxide. We then look at vertical excitations for small organic molecules up to the size of butadiene where the SA-MCCI energies and oscillator strengths are compared with CASPT2 values [M. Schreiber, M. R. Silva-Junior, S. P. A. Sauer, and W. Thiel, J. Chem. Phys., 128, 134110 (2008)]. We finally see if the SA-MCCI results for these excitation energies can be improved by using MCCIPT2 with approximate natural orbitals when the PT2 space is not onerously large.

Section: Mccipt2 Results With Approximate Natural Orbitalsmentioning

confidence: 99%

“…12. There it was shown that the standard deviation of the difference in energies (σ ∆E ) can be used as a measure of the error between two potential curves.…”

Section: A H 3 Equilateral Trianglementioning

confidence: 99%

State-averaged Monte Carlo configuration interaction applied to electronically excited states

Coe

Paterson

2013

Self Cite

“…[31][32][33][34][35][36][37] We use a version of the program, which has been modified to treat the complex symmetric generalized eigenvalue problem that arises when adding the CAP to the many-electron Coulomb Hamiltonian. The CAP is introduced as a one-body operator describing the extended device region and its interaction with the electrodes into the CI Hamiltonian.…”

Section: F Complex Symmetric CI Problemmentioning

confidence: 99%

Quasiparticle energies and lifetimes in a metallic chain model of a tunnel junction

Szepieniec

Yeriskin

Greer

2013

As electronics devices scale to sub-10 nm lengths, the distinction between "device" and "electrodes" becomes blurred. Here, we study a simple model of a molecular tunnel junction, consisting of an atomic gold chain partitioned into left and right electrodes, and a central "molecule." Using a complex absorbing potential, we are able to reproduce the single-particle energy levels of the device region including a description of the effects of the semi-infinite electrodes. We then use the method of configuration interaction to explore the effect of correlations on the system's quasiparticle peaks. We find that when excitations on the leads are excluded, the device's highest occupied molecular orbital and lowest unoccupied molecular orbital quasiparticle states when including correlation are bracketed by their respective values in the Hartree-Fock (Koopmans) and ΔSCF approximations. In contrast, when excitations on the leads are included, the bracketing property no longer holds, and both the positions and the lifetimes of the quasiparticle levels change considerably, indicating that the combined effect of coupling and correlation is to alter the quasiparticle spectrum significantly relative to an isolated molecule.

“…Within the context of MCCI calculations, the perturbative estimates have been used to pre-screen the randomly chosen configurations to be included within the CI vector prior to a matrix diagonalization step. 35 Recently Coe and Paterson 26 have introduced a hybrid method which begins by first converging an MCCI calculation to a given configuration threshold selection value, then continues by adaptively refining the CI vector in a scheme similar to Harrison's algorithm.…”

Section: B MCCI + a K Estimatesmentioning

confidence: 99%

“…MCCI has been applied to the calculation of electronic spectra, charge transport in molecules, dissociation energies, multipole moments, ionization energies, and electron affinities. [23][24][25][26][27] In this way, the need for working with large CI vectors is avoided while a large fraction (for reasonable convergence parameters and 10-20 electrons, typically >96%) of the total correlation energy is readily obtained. The ability of the MCCI method to recover static correlation to qualitatively describe potential energy dissociation curves, as well as the ability to describe dynamic correlations for accurate estimates of dissociation energies, is studied.…”

Section: Introductionmentioning

confidence: 99%

Monte Carlo configuration interaction with perturbation corrections for dissociation energies of first row diatomic molecules: C2, N2, O2, CO, and NO

Kelly

Perera

Bartlett

et al. 2014

Dissociation energies for the diatomic molecules C2, N2, O2, CO, and NO are estimated using the Monte Carlo configuration interaction (MCCI) and augmented by a second order perturbation theory correction. The calculations are performed using the correlation consistent polarized valence “triple zeta” atomic orbital basis and resulting dissociation energies are compared to coupled cluster calculations including up to triple excitations (CCSDT) and Full Configuration Interaction Quantum Monte Carlo (FCIQMC) estimates. It is found that the MCCI method readily describes the correct behavior for dissociation for the diatomics even when capturing only a relatively small fraction (∼80%) of the correlation energy. At this level only a small number of configurations, typically O(103) from a FCI space of dimension O(1014), are required to describe dissociation. Including the perturbation correction to the MCCI estimates, the difference in dissociation energies with respect to CCSDT ranges between 1.2 and 3.1 kcal/mol, and the difference when comparing to FCIQMC estimates narrows to between 0.5 and 1.9 kcal/mol. Discussions on MCCI's ability to recover static and dynamic correlations and on the form of correlations in the electronic configuration space are presented.