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1.
J Chem Theory Comput ; 13(8): 3814-3828, 2017 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-28628314

RESUMO

We study the vertical and adiabatic ionization potentials and electron affinities of bare and hydroxylated TiO2 nanoclusters, as well as their fundamental gap and exciton binding energy values, to understand how the clusters' electronic properties change as a function of size and hydroxylation. In addition, we have employed a range of many-body methods; including G0W0, qsGW, EA/IP-EOM-CCSD, and DFT (B3LYP, PBE), to compare the performance and predictions of the different classes of methods. We demonstrate that, for bare clusters, all many-body methods predict the same trend with cluster size. The highest occupied and lowest unoccupied DFT orbitals follow the same trends as the electron affinity and ionization potentials predicted by the many-body methods, but are generally far too shallow and deep respectively in absolute terms. In contrast, the ΔDFT method is found to yield values in the correct energy window. However, its predictions depend upon the functional used and do not necessarily follow trends based on the many-body methods. Adiabatic potentials are predicted to be similar to their vertical counterparts and holes found to be trapped more strongly than excess electrons. The effect of hydroxylation on the clusters is to open up both the optical and fundamental gap. Finally, a simple microscopic explanation for the observed trends with cluster size and upon hydroxylation is proposed in terms of the onsite electrostatic potential.

2.
ACS Nano ; 10(7): 7008-22, 2016 07 26.
Artigo em Inglês | MEDLINE | ID: mdl-27299676

RESUMO

Understanding how additives interact and segregate within bulk heterojunction (BHJ) thin films is critical for exercising control over structure at multiple length scales and delivering improvements in photovoltaic performance. The morphological evolution of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blends that are commensurate with the size of a BHJ thin film is examined using petascale coarse-grained molecular dynamics simulations. Comparisons between two-component and three-component systems containing short P3HT chains as additives undergoing thermal annealing demonstrate that the short chains alter the morphology in apparently useful ways: they efficiently migrate to the P3HT/PCBM interface, increasing the P3HT domain size and interfacial area. Simulation results agree with depth profiles determined from neutron reflectometry measurements that reveal PCBM enrichment near substrate and air interfaces but a decrease in that PCBM enrichment when a small amount of short P3HT chains are integrated into the BHJ blend. Atomistic simulations of the P3HT/PCBM blend interfaces show a nonmonotonic dependence of the interfacial thickness as a function of number of repeat units in the oligomeric P3HT additive, and the thiophene rings orient parallel to the interfacial plane as they approach the PCBM domain. Using the nanoscale geometries of the P3HT oligomers, LUMO and HOMO energy levels calculated by density functional theory are found to be invariant across the donor/acceptor interface. These connections between additives, processing, and morphology at all length scales are generally useful for efforts to improve device performance.

3.
J Chem Phys ; 144(14): 144101, 2016 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-27083702

RESUMO

In this paper, we report on the development of a parallel implementation of the coupled-cluster (CC) Green function formulation (GFCC) employing single and double excitations in the cluster operator (GFCCSD). A key aspect of this work is the determination of the frequency dependent self-energy, Σ(ω). The detailed description of the underlying algorithm is provided, including approximations used that preserve the pole structure of the full GFCCSD method, thereby reducing the computational costs while maintaining an accurate character of methodology. Furthermore, for systems with strong local correlation, our formulation reveals a diagonally dominate block structure where as the non-local correlation increases, the block size increases proportionally. To demonstrate the accuracy of our approach, several examples including calculations of ionization potentials for benchmark systems are presented and compared against experiment.

4.
J Chem Phys ; 144(1): 014101, 2016 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-26747795

RESUMO

The Sign Learning Kink (SiLK) based Quantum Monte Carlo (QMC) method is used to calculate the ab initio ground state energies for multiple geometries of the H2O, N2, and F2 molecules. The method is based on Feynman's path integral formulation of quantum mechanics and has two stages. The first stage is called the learning stage and reduces the well-known QMC minus sign problem by optimizing the linear combinations of Slater determinants which are used in the second stage, a conventional QMC simulation. The method is tested using different vector spaces and compared to the results of other quantum chemical methods and to exact diagonalization. Our findings demonstrate that the SiLK method is accurate and reduces or eliminates the minus sign problem.

5.
J Chem Phys ; 143(22): 224301, 2015 Dec 14.
Artigo em Inglês | MEDLINE | ID: mdl-26671369

RESUMO

The photophysics of the Green Fluorescent Protein (GFP) chromophore is critically dependent on its local structure and on its environment. Despite extensive experimental and computational studies, there remain many open questions regarding the key fundamental variables that govern this process. One outstanding problem is the role of autoionization as a possible relaxation pathway of the excited state under different environmental conditions. This issue is considered in our work through combined experimental and theoretical studies of microsolvated clusters of the deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone anion (HBDI(-)), an analog of the GFP chromophore. Through selective generation of microsolvated structures of predetermined size and subsequent analysis of experimental photoelectron spectra by high level ab initio methods, we are able to precisely identify the structure of the system, establish the accuracy of theoretical data, and provide reliable description of auto-ionization process as a function of hydrogen-bonding environment. Our study clearly illustrates the first few water molecules progressively stabilize the excited state of the chromophore anion against the autodetached neutral state, which should be an important trait for crystallographic water molecules in GFPs that has not been fully explored to date.


Assuntos
Compostos de Benzilideno/química , Elétrons , Proteínas de Fluorescência Verde/química , Imidazóis/química , Espectroscopia Fotoeletrônica , Teoria Quântica , Água/química , Ânions/química , Estrutura Molecular
6.
J Chem Phys ; 141(7): 074304, 2014 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-25149783

RESUMO

In both molecular and periodic solid-state systems there is a need for the accurate determination of the ionization potential and the electron affinity for systems ranging from light harvesting polymers and photocatalytic compounds to semiconductors. The development of a Green's function approach based on the coupled cluster (CC) formalism would be a valuable tool for addressing many properties involving many-body interactions along with their associated correlation functions. As a first step in this direction, we have developed an accurate and parallel efficient approach based on the equation of motion-CC technique. To demonstrate the high degree of accuracy and numerical efficiency of our approach we calculate the ionization potential and electron affinity for C60 and C70. Accurate predictions for these molecules are well beyond traditional molecular scale studies. We compare our results with experiments and both quantum Monte Carlo and GW calculations.

7.
J Phys Chem A ; 118(39): 9087-93, 2014 Oct 02.
Artigo em Inglês | MEDLINE | ID: mdl-24905471

RESUMO

In this paper we discuss the application of novel parallel implementation of the coupled cluster (CC) and equation-of-motion coupled cluster methods (EOMCC) in calculations of excitation energies of triplet states in ß-carotene. Calculated excitation energies are compared with experimental data, where available. We also provide a detailed description of the new parallel algorithms for iterative CC and EOMCC models involving singles and doubles excitations.


Assuntos
Modelos Químicos , beta Caroteno/química , Algoritmos , Movimento (Física) , Teoria Quântica
8.
J Chem Phys ; 138(20): 204114, 2013 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-23742461

RESUMO

The universal state selective (USS) multireference approach is used to construct new energy functionals which offer a possibility of bridging single and multireference coupled cluster theories (SR/MRCC). These functionals, which can be used to develop iterative and non-iterative approaches, utilize a special form of the trial wavefunctions, which assure additive separability (or size-consistency) of the USS energies in the non-interacting subsystem limit. When the USS formalism is combined with approximate SRCC theories, the resulting formalism can be viewed as a size-consistent version of the method of moments of coupled cluster equations employing a MRCC trial wavefunction. Special cases of the USS formulations, which utilize single reference state specific CC [V. V. Ivanov, D. I. Lyakh, and L. Adamowicz, Phys. Chem. Chem. Phys. 11, 2355 (2009)] and tailored CC [T. Kinoshita, O. Hino, and R. J. Bartlett, J. Chem. Phys. 123, 074106 (2005)] expansions are also discussed.

9.
J Phys Chem A ; 117(23): 4988-95, 2013 Jun 13.
Artigo em Inglês | MEDLINE | ID: mdl-23675905

RESUMO

Enthalpies of formation for the ThX4 and UX4 (X = F, Cl, OH) species have been investigated with density functional theory and coupled-cluster methods. ThX4 molecules are all confirmed as tetrahedral, while all UX4 molecules are predicted to adopt D2d symmetry using density functional theory. Multireference coupled cluster approaches confirm the D2d symmetry for UF4. The bonding is mostly ionic, and predicted formation energies for the halogen species show good agreement with experiment. Our calculated hydration energy of UF4 (-54.0 kcal/mol) is in very good agreement with the experimental data (-54.8 kcal/mol). We predict CCSD(T) formation energies of ΔfG[U(OH)4(g)] = -286.3 kcal/mol and ΔfG[U(OH)4(aq)] = -318.7 kcal/mol. ΔfG[U(OH)4(aq)] is 21 kcal/mol less stable than the established experimental thermodynamic data.


Assuntos
Compostos Organometálicos/química , Teoria Quântica , Termodinâmica , Tório/química , Urânio/química
10.
J Chem Theory Comput ; 9(4): 1949-57, 2013 Apr 09.
Artigo em Inglês | MEDLINE | ID: mdl-26583545

RESUMO

A novel parallel algorithm for noniterative multireference coupled cluster (MRCC) theories, which merges recently introduced reference-level parallelism (RLP) [Bhaskaran-Nair, K.; Brabec, J.; Aprà, E.; van Dam, H. J. J.; Pittner, J.; Kowalski, K. J. Chem. Phys.2012, 137, 094112] with the possibility of accelerating numerical calculations using graphics processing units (GPUs) is presented. We discuss the performance of this approach applied to the MRCCSD(T) method (iterative singles and doubles and perturbative triples), where the corrections due to triples are added to the diagonal elements of the MRCCSD effective Hamiltonian matrix. The performance of the combined RLP/GPU algorithm is illustrated on the example of the Brillouin-Wigner (BW) and Mukherjee (Mk) state-specific MRCCSD(T) formulations.

12.
J Chem Phys ; 137(17): 171101, 2012 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-23145708

RESUMO

The concept of the model space underlying multireference coupled-cluster (MRCC) formulations is a powerful tool to deal with complex correlation effects for various electronic states. Here, we demonstrate that iterative state-specific MRCC methods (SS-MRCC) based on properly defined model spaces can be used to describe core-level excited states even when Hartree-Fock orbitals are utilized. We show that the SS-MRCC models with single and double excitations are comparable in accuracy to high-level single reference equation-of-motion coupled cluster (EOMCC) formalism.

13.
J Chem Phys ; 137(9): 094112, 2012 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-22957560

RESUMO

In this paper we discuss the performance of the non-iterative state-specific multireference coupled cluster (SS-MRCC) methods accounting for the effect of triply excited cluster amplitudes. The corrections to the Brillouin-Wigner and Mukherjee's MRCC models based on the manifold of singly and doubly excited cluster amplitudes (BW-MRCCSD and Mk-MRCCSD, respectively) are tested and compared with exact full configuration interaction results for small systems (H(2)O, N(2), and Be(3)). For the larger systems (naphthyne isomers) the BW-MRCC and Mk-MRCC methods with iterative singles, doubles, and non-iterative triples (BW-MRCCSD(T) and Mk-MRCCSD(T)) are compared against the results obtained with single reference coupled cluster methods. We also report on the parallel performance of the non-iterative implementations based on the use of processor groups.

14.
J Chem Phys ; 134(15): 154106, 2011 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-21513374

RESUMO

A new version of the multireference Mukherjee's coupled cluster method with perturbative triexcitations has been formulated, which is based on the uncoupled approximation applied to the triples equation. In contrast to the method developed by Evangelista et al. [J. Chem. Phys. 132, 074107 (2010)], the proposed approach does not require to solve the equation for T(3) amplitudes iteratively, yet yields results of essentially the same quality. The method, abbreviated as MR MkCCSD(Tu), has been implemented in the ACES II program package and its assessment has been performed on the BeH(2) model and on the tetramethyleneethane molecule.

15.
J Chem Phys ; 133(13): 134106, 2010 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-20942522

RESUMO

We have developed the uncoupled version of multireference Mukherjee's coupled cluster method with connected triexcitations. The method has been implemented in ACES II program package. The agreement between the uncoupled and the standard version of Mukherjee's multireference coupled cluster method has been reported previously at the singles and doubles level by Das et al. [J. Mol. Struct.: THEOCHEM 79, 771 (2006); Chem. Phys. 349, 115 (2008)]. The aim of this article is to investigate this method further, in order to establish how its performance changes with the size of the basis set, size of the model space, multireference character of different molecules, and inclusion of connected triple excitations. Assessment of the new method has been performed on the singlet methylene, potential energy curve of fluorine molecule, and third b (1)Σ(g)(+) electronic state of oxygen molecule.

16.
J Chem Phys ; 132(15): 154105, 2010 Apr 21.
Artigo em Inglês | MEDLINE | ID: mdl-20423166

RESUMO

We have formulated the multireference Mukherjee's coupled clusters method with triexcitations (MR MkCCSDT) in the linked version and implemented it in the ACES II program package. The assessment of the new method has been performed on the first three electronic states of the oxygen molecule, on studies of singlet-triplet gap in methylene and twisted ethylene, where a comparison with other multireference CC treatments and with experimental data is available. The MR MkCCSDT results show accuracy comparable to which can be achieved with CCSDT in single reference cases. Comparison of the previously developed MkCCSD(T) method with MkCCSDT as a reference suggests, that MkCCSD(T) might be a promising candidate for an accurate treatment of systems where the static correlation plays an important role, at least for situations where small model spaces are sufficient.

17.
J Chem Phys ; 129(18): 184105, 2008 Nov 14.
Artigo em Inglês | MEDLINE | ID: mdl-19045384

RESUMO

We have formulated and implemented the multireference Mukherjee's coupled cluster method with connected singles, doubles, and perturbative triples [MR MkCCSD(T)] in the ACES II program package. Assessment of the new method has been performed on the first three electronic states of the oxygen molecule and on the automerization barrier of cyclobutadiene, where a comparison with other multireference CC treatments and with experimental data where available. The MR MkCCSD(T) method seems to be a promising candidate for an accurate, yet computationally tractable, treatment of systems where the static correlation plays an important role.

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