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Current-Driven Desorption at the Organic Molecule–Semiconductor Interface: Cyclopentene on Si(100)
Published in Tamar Seideman, Current-Driven Phenomena in Nanoelectronics, 2016
N. L. Yoder, R. Jorn, C.-C. Kaun, T. Seideman, M. C. Hersam
Electronic structure calculations were carried out using density functional theory (DFT) with the B3LYP hybrid exchange-correlation functional within Q-Chem.53 Three Pople-style Gaussian basis sets were contrasted: STO-3G, 6-31G*, and 6-311G**, and good agreement was found for bond lengths and angles in the neutral geometry. Selected configurations were also computed at the coupled-cluster level of theory with singly and doubly excited determinants (CCSD) in order to test the reliability of the B3LYP/6-311G approximation for the system at hand and place an error bar on our results. The silicon surface was approximated
Metal sandwich and ion complexes in cyclacene nanobelts
Published in Molecular Physics, 2023
Density functional theory calculations were performed using a developmental version of the Q-Chem 5 quantum chemical software package.[14] Minimum energy geometries were calculated for isomers of the Hückel-[n]cyclacenes (n = 5-8), with zero, one, or two Dewar benzenoid structures present, both with and without halide and alkali metal ions placed at the approximate centre of the belt cavities. The different positions that two Dewar benzenoids can occupy relative to each other in these belts will lead to structures with different molecular properties. However, previous theoretical studies have suggested that the isomers with the two Dewar benzenoids located on opposite sides of the molecule have lower energies, and these structures are exclusively considered here.[8] Geometry optimizations for these species were carried out using unrestricted Kohn–Sham density functional theory with the M06-2X exchange–correlation functional and the 6-311++G(d,p) basis set,[15] and the geometries were confirmed as being minima by the absence of imaginary harmonic nuclear vibrational frequencies. Vibrational zero-point energy (ZPE) corrections were also calculated at this level of theory.
Nicholas A. Besley (1972–2021)
Published in Molecular Physics, 2023
Jonathan D. Hirst, Andrew M. Teale, Anthony J. Stace, Peter J. Knowles
With Peter, Nick deepened his knowledge of quantum chemistry, developing the theory of intracules [12,14,15,17,18]: two-electron functions that are well suited to studying inter-electronic interactions and the effects of electron correlation. The study of intracules [23,27,30] was the basis for a prestigious 5-year EPSRC Advanced Research Fellowship, which Nick was awarded in 2002. He became a significant contributor to the quantum chemistry software suite, Q-Chem [11,29,73,132], notably the maximum overlap method (MOM) for self-consistent field calculations of excited states [38], which arose from a sabbatical visit in 2007 with Peter Gill, who had moved to the Australian National University. The MOM helps maintain the target non-Aufbau orbital occupations and is a practical implementation of the so-called Δ-SCF method. It has found widespread utility, as reflected in several hundred citations of the paper. One of its more recent applications has been to ultrafast nonadiabatic dynamics [120].
The symmetric quasi-classical model using on-the-fly time-dependent density functional theory within the Tamm–Dancoff approximation
Published in Molecular Physics, 2023
Justin J. Talbot, Martin Head-Gordon, Stephen J. Cotton
The symmetric quasi-classical model for quantising classical Meyer–Miller vibronic dynamics is an efficient, and often quite accurate framework for performing ab inito molecular dynamics for electronically nonadiabatic processes, such as vibrational-DOF enhanced electronic energy transfer dynamics and the role that nonadiabatic energy transfer has on geometric and other properties. Here, what has been developed for general use is an implementation of the SQC/MM model using ‘on-the-fly’ TDDFT/TDA within the widely available Q-Chem quantum chemistry software package, including the efficient implementation of new algorithms that improve the compute cost when evaluating analytic nuclear gradients and first-order derivative coupling vectors. In particular, new digestion routines were proposed that contract the full set of density matrices with a common set of integrals and integral derivatives which were shown to speedup the calculations by a factor of four compared with the brute force method. The efficiency gains that were achieved as a result of these new algorithms should aid in simulating realistic time-scales of nonadiabatic dynamics in moderately sized molecular systems.