Reference Dependence of the Two-determinant Coupled-cluster Method for Triplet and Open-shell Singlet States of Biradical Molecules

We study the performance of the two-determinant (TD) coupled-cluster (CC) method which, unlike conventional ground-state single-reference (SR) CC methods, can, in principle, provide a naturally spin-adapted treatment of the lowest-lying open-shell singlet (OSS) and triplet electronic states. Various choices for the TD-CC reference orbitals are considered, including those generated by the multi-configurational self-consistent field method. Comparisons are made with the results of high-level SR-CC, equation-of-motion (EOM) CC, and multi-reference EOM calculations performed on a large test set of over 100 molecules with low-lying OSS states. It is shown that in cases where the EOMCC reference function is poorly described, TD-CC can provide a significantly better quantitative description of OSS total energies and OSS-triplet splittings

Multireference equation of motion coupled cluster benchmark study of magnetic model systems

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.comptc.2018.03.009 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license https://creativecommons.org/licenses/by-nc-nd/4.0/The multireference equation of motion coupled cluster (MREOM-CC) approach including spin-orbit coupling is applied to magnetic model systems FArO, FArOF and FArFOH, and compared to benchmark multireference Configuration Interaction (MRCI) approaches in the case of FArO. In the MREOM calculations, convenient high-spin states provide reference reduced density matrices, that are used to obtain a sequence of similarity transformations of the Hamiltonian. All low-lying magnetic states are obtained subsequently from a compact diagonalization of the transformed Hamiltonian. The accuracy of MREOM is shown to be comparable to MRCI+Q, but the approach is significantly more efficient for systems with a large number of electronic states. Moreover, MREOM is nearly size-consistent and this allows one to perform meaningful calculations of the strength of the magnetic coupling in the weak coupling limit.Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery gran

2nd-order many-body perturbation approximations to the coupled-cluster Greens-function

The time‐consuming step in coupled cluster Green’s function or equivalently equation of motion coupled cluster calculations of ionization potentials is the solution of the CCSD equations. We investigate here the accuracy that can be obtained if the CCSD coefficients are replaced by their MBPT(2) analogs. We discuss some additional diagonal approximations that might prove especially useful in polymer calculations, and compare with traditional Green’s function calculations based on a second order approximation to the irreducible self‐energy

National consensus on a new resectability classification for perihilar cholangiocarcinoma - A modified Delphi method

Background: Currently, no practical definition of potentially resectable, borderline or unresectable perihilar cholangiocarcinoma (pCCA) is available. Aim of this study was to define criteria to categorize patients for use in a future neoadjuvant or induction therapy study. Method: Using the modified DELPHI method, hepatobiliary surgeons from all tertiary referral centers in the Netherlands were invited to participate in this study. During five online meetings, predefined factors determining resectability and additional factors regarding surgical resectability and operability were discussed. Results: The five online meetings resulted in 52 statements. After two surveys, consensus was reached in 63% of the questions. The main consensus included a definition regarding potential resectability. 1) Clearly resectable: no vascular involvement (≤90°) of the future liver remnant (FLR) and expected feasibility of radical biliary resection. 2) Clearly unresectable: non-reconstructable venous and/or arterial involvement of the FLR or no feasible radical biliary resection. 3) Borderline resectable: all patients between clearly resectable and clearly unresectable disease. Conclusion: This DELPHI study resulted in a practical and applicable resectability, or more accurate, an explorability classification, which can be used to categorize patients for use in future neoadjuvant therapy studies.</p

A path integral methodology for obtaining thermodynamic properties of nonadiabatic systems using Gaussian mixture distributions

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Raymond, N., Iouchtchenko, D., Roy, P.-N., & Nooijen, M. (2018). A path integral methodology for obtaining thermodynamic properties of nonadiabatic systems using Gaussian mixture distributions. The Journal of Chemical Physics, 148(19), 194110 and may be found at https://doi.org/10.1063/1.5025058We introduce a new path integral Monte Carlo method for investigating nonadiabatic systems in thermal equilibrium and demonstrate an approach to reducing stochastic error. We derive a general path integral expression for the partition function in a product basis of continuous nuclear and discrete electronic degrees of freedom without the use of any mapping schemes. We separate our Hamiltonian into a harmonic portion and a coupling portion; the partition function can then be calculated as the product of a Monte Carlo estimator (of the coupling contribution to the partition function) and a normalization factor (that is evaluated analytically). A Gaussian mixture model is used to evaluate the Monte Carlo estimator in a computationally efficient manner. Using two model systems, we demonstrate our approach to reduce the stochastic error associated with the Monte Carlo estimator. We show that the selection of the harmonic oscillators comprising the sampling distribution directly affects the efficiency of the method. Our results demonstrate that our path integral Monte Carlo method’s deviation from exact Trotter calculations is dominated by the choice of the sampling distribution. By improving the sampling distribution, we can drastically reduce the stochastic error leading to lower computational cost.Natural Sciences and Engineering Research Council of Canada (NSERC) Ontario Ministry of Research and Innovation (MRI) Canada Foundation for Innovation (CFI) Canada Research Chair progra

State Selective Equation of Motion Coupled Cluster Theory: Some Preliminary Results

Abstract: A multireference variant of coupled cluster theory is described that applies to systems that can qualitatively be described by deleting two electrons from a closed shell determinant, for example biradicals, single bond breaking processes, or valence excited states. The theory can be generalized to arbitrary open-shell systems and takes a form that is akin to equation-of-motion coupled cluster theory, but where all wave function parameters are explicitly optimized for the state of interest. The implementation of the present methods was accomplished in an automated fashion using the recently developed Automatic Program Generator (APG). We present benchmark results for the O2 and F2 molecules and investigate the behaviour of a number of closely related variants within the same general framework