| 1. |
- Epifanovsky, Evgeny, et al.
(författare)
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Software for the frontiers of quantum chemistry : An overview of developments in the Q-Chem 5 package
- 2021
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Ingår i: Journal of Chemical Physics. - : AMER INST PHYSICS. - 0021-9606 .- 1089-7690. ; 155:8
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Tidskriftsartikel (refereegranskat)abstract
- This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.
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| 2. |
- Gozem, Samer, et al.
(författare)
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Shape of Multireference, Equation-of-Motion Coupled-Cluster, and Density Functional Theory Potential Energy Surfaces at a Conical Intersection
- 2014
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 10:8, s. 3074-3084
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Tidskriftsartikel (refereegranskat)abstract
- We report and characterize ground-state and excited-state potential energy profiles using a variety of electronic structure methods along a loop lying on the branching plane associated with a conical intersection (Cl) of a reduced retinal model, the penta-2,4-dieniminium cation (PSB3). Whereas the performance of the equation-of-motion coupled-duster, density functional theory, and multireference methods had been tested along the excited- and ground-state paths of PSB3 in our earlier work, the ability of these methods to correctly describe the potential energy surface shape along a CI branching plane has not yet been investigated. This is the focus of the present contribution. We find, in agreement with earlier studies by others, that standard time-dependent DFT (TDDFT) does not yield the correct two-dimensional (i.e., conical) crossing along the branching plane but rather a one-dimensional (i.e., linear) crossing along the same plane. The same type of behavior is found for SS-CASPT2(IPEA=0), SS-CASPT2(IPEA=0.25), spin-projected SF-TDDFT, EOM-SF-CCSD, and, finally, for the reference MRCISD+Q method. In contrast, we found that MRCISD, CASSCF, MS-CASPT2(IPEA=0), MS-CASPT2(IPEA=0.25), XMCQDPT2, QD-NEVPT2, non-spin-projected SF-TDDFT, and SI-SA-REKS yield the expected conical crossing. To assess the effect of the different crossing topologies (i.e., linear or conical) on the PSB3 photoisomerization efficiency, we discuss the results of 100 semiclassical trajectories computed by CASSCF and SS-CASPT2(IPEA=0.25) for a PSB3 derivative. We show that for the same initial conditions, the two methods yield similar dynamics leading to isomerization quantum yields that differ by only a few percent.
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| 3. |
- Krylov, Anna I., et al.
(författare)
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What Is the Price of Open-Source Software?
- 2015
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 6:14, s. 2751-2754
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 4. |
- Orms, Natalie, et al.
(författare)
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Characterizing Bonding Patterns in Diradicals and Triradicals by Density-Based Wave Function Analysis : A Uniform Approach
- 2018
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 14:2, s. 638-648
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Tidskriftsartikel (refereegranskat)abstract
- Density-based wave function analysis enables unambiguous comparisons of the electronic structure computed by different methods and removes ambiguity of orbital choices. We use this tool to investigate the performance of different spin flip methods for several prototypical diradicals and triradicals. In contrast to previous calibration studies that focused on energy gaps between high- and low spin-states, we focus on the properties of the underlying wave functions, such as the number of effectively unpaired electrons. Comparison of different density functional and wave function theory results provides insight into the performance of the different methods when applied to strongly correlated systems such as polyradicals. We show that canonical molecular orbitals for species like large copper-containing diradicals fail to correctly represent the underlying electronic structure due to highly non-Koopmans character, while density-based analysis of the same wave function delivers a clear picture of the bonding pattern.
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| 5. |
- Zuev, Dmitry, et al.
(författare)
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Electronic structure of the two isomers of the anionic form of p-coumaric acid chromophore
- 2011
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 134:3, s. 034310-
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Tidskriftsartikel (refereegranskat)abstract
- A theoretical study of the electronic structure of the photoactive yellow protein (PYP) model chromophore, para-coumaric acid (p-CA), is presented. Electronically excited states of the phenolate and carboxylate isomers of the deprotonated p-CA are characterized by high-level ab initio methods including state-specific and multistate multireference pertrubation theory (SS-CASPT2, and MS-CASPT2), equation-of-motion coupled-cluster methods with single and double substitutions (EOM-CCSD) and with an approximate account of triple excitations (CC3). We found that the two isomers have distinctly different patterns of ionization and excitation energies. Their excitation energies differ by more than 1 eV, in contradiction to the experimental report [Rocha-Rinza et al., J. Phys. Chem. A 113, 9442 (2009)]. The calculations confirm metastable (autoionizing) character of the valence excited states of both phenolate and carboxylate isomers of p-CA− in the gas phase. The type of resonance is different in the two forms. In the phenolate, the excited state lies above the detachment continuum (a shape resonance), whereas in the carboxylate the excited π→π* state lies below the π-orbital ionization continuum, but is above the states derived from ionization from three other orbitals (Feshbach resonance). The computed oscillator strength of the bright electronic state in the phenolate is higher than in the carboxylate, in agreement with Hückel's model predictions. The analysis of photofragmentation channels shows that the most probable products for the methylated derivatives of the phenolate and carboxylate forms of p-CA− are CH3, CH2O and CH3, CH2O, CO2, respectively, thus suggesting an experimental probe that may discriminate between the two isomers.
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