Wesley D. Allen

Bio: Wesley D. Allen is an academic researcher from University of Georgia. The author has contributed to research in topics: Ab initio & Coupled cluster. The author has an hindex of 53, co-authored 136 publications receiving 9582 citations. Previous affiliations of Wesley D. Allen include University of California, Los Angeles & Lawrence Berkeley National Laboratory.

Psi4: an open-source ab initio electronic structure program

Abstract: The Psi4 program is a new approach to modern quantum chemistry, encompassing Hartree–Fock and density-functional theory to configuration interaction and coupled cluster. The program is written entirely in C++ and relies on a new infrastructure that has been designed to permit high-efficiency computations of both standard and emerging electronic structure methods on conventional and high-performance parallel computer architectures. Psi4 offers flexible user input built on the Python scripting language that enables both new and experienced users to make full use of the program's capabilities, and even to implement new functionality with moderate effort. To maximize its impact and usefulness, Psi4 is available through an open-source license to the entire scientific community. © 2011 John Wiley & Sons, Ltd.

In pursuit of the ab initio limit for conformational energy prototypes

Abstract: The convergence of ab initio predictions to the one- and n-particle limits has been systematically explored for several conformational energy prototypes: the inversion barriers of ammonia, water, and isocyanic acid, the torsional barrier of ethane, the E/Z rotamer separation of formic acid, and the barrier to linearity of silicon dicarbide. Explicit ab initio results were obtained with atomic-orbital basis sets as large as [7s6p5d4f3g2h1i/6s5p4d3f2g1h] and electron correlation treatments as extensive as fifth-order Mo/ller–Plesset perturbation theory (MP5), the full coupled-cluster method through triple excitations (CCSDT), and Brueckner doubles theory including perturbational corrections for both triple and quadruple excitations [BD(TQ)]. Subsequently, basis set and electron correlation extrapolation schemes were invoked to gauge any further variations in arriving at the ab initio limit. Physical effects which are tacitly neglected in most theoretical work have also been quantified by computations of non...

A hierarchy of homodesmotic reactions for thermochemistry.

Abstract: Chemical equations that balance bond types and atom hybridization to different degrees are often used in computational thermochemistry, for example, to increase accuracy when lower levels of theory are employed. We expose the widespread confusion over such classes of equations and demonstrate that the two most widely used definitions of "homodesmotic" reactions are not equivalent. New definitions are introduced, and a consistent hierarchy of reaction classes (RC1-RC5) for hydrocarbons is constructed: isogyric (RC1) superset of isodesmic (RC2) superset of hypohomodesmotic (RC3) superset of homodesmotic (RC4) superset of hyperhomodesmotic (RC5). Each of these successively conserves larger molecular fragments. The concept of isodesmic bond separation reactions is generalized to all classes in this hierarchy, providing a unique sectioning of a given molecule for each reaction type. Several ab initio and density functional methods are applied to the bond separation reactions of 38 hydrocarbons containing five or six carbon atoms. RC4 and RC5 reactions provide bond separation enthalpies with errors consistently less than 0.4 kcal mol(-1) across a wide range of theoretical levels, performing significantly better than the other reaction types and far superior to atomization routes. Our recommended bond separation reactions are demonstrated by determining the enthalpies of formation (at 298 K) of 1,3,5-hexatriyne (163.7 +/- 0.4 kcal mol(-1)), 1,3,5,7-octatetrayne (217.5 +/- 0.6 kcal mol(-1)), the larger polyynes C(10)H(2) through C(26)H(2), and an infinite acetylenic carbon chain.

The heat of formation of NCO

Abstract: The heat of formation of NCO has been determined rigorously by state‐of‐the‐art ab initio electronic structure methods, including Mo/ller–Plesset perturbation theory from second through fifth order (MP2–MP5) and coupled‐cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. Five independent reactions were investigated to establish a consistent value for ΔHf,0○(NCO): (a) HNCO(X 1A’)→H(2S)+NCO(2Π), (b) HNCO(X 1A’)→H++NCO−, (c) N(4S)+CO→NCO(2Π), (d) HCN+O(3P)→H(2S)+NCO(2Π), and (e) NH(3Σ−)+CO→H(2S)+NCO(2Π). The one‐particle basis sets employed in the study were comprised of as many as 377 contracted Gaussian functions and ranged in quality from [4s2p1d] to [14s9p6d4f] on the (C,N,O) atoms and from [2s1p] to [8s6p4d] on hydrogen. After the addition of bond additivity corrections evaluated from related reactions of precisely known thermochemistry, all five approaches were found to converge on the value ΔHf,0○(NCO)=31.4(5) kcal mol−1. Appurtenant refi...

Toward subchemical accuracy in computational thermochemistry: focal point analysis of the heat of formation of NCO and [H,N,C,O] isomers.

Abstract: In continuing pursuit of thermochemical accuracy to the level of 0.1 kcal mol−1, the heats of formation of NCO, HNCO, HOCN, HCNO, and HONC have been rigorously determined using state-of-the-art ab initio electronic structure theory, including conventional coupled cluster methods [coupled cluster singles and doubles (CCSD), CCSD with perturbative triples (CCSD(T)), and full coupled cluster through triple excitations (CCSDT)] with large basis sets, conjoined in cases with explicitly correlated MP2-R12/A computations. Limits of valence and all-electron correlation energies were extrapolated via focal point analysis using correlation consistent basis sets of the form cc-pVXZ (X=2–6) and cc-pCVXZ (X=2–5), respectively. In order to reach subchemical accuracy targets, core correlation, spin-orbit coupling, special relativity, the diagonal Born–Oppenheimer correction, and anharmonicity in zero-point vibrational energies were accounted for. Various coupled cluster schemes for partially including connected quadrupl...

QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

Abstract: QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

Harmonic Vibrational Frequencies: An Evaluation of Hartree−Fock, Møller−Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors

Abstract: Scaling factors for obtaining fundamental vibrational frequencies, low-frequency vibrations, zero-point vibrational energies (ZPVE), and thermal contributions to enthalpy and entropy from harmonic frequencies determined at 19 levels of theory have been derived through a least-squares approach. Semiempirical methods (AM1 and PM3), conventional uncorrelated and correlated ab initio molecular orbital procedures [Hartree−Fock (HF), Moller−Plesset (MP2), and quadratic configuration interaction including single and double substitutions (QCISD)], and several variants of density functional theory (DFT: B-LYP, B-P86, B3-LYP, B3-P86, and B3-PW91) have been examined in conjunction with the 3-21G, 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-311G(d,p), and 6-311G(df,p) basis sets. The scaling factors for the theoretical harmonic vibrational frequencies were determined by a comparison with the corresponding experimental fundamentals utilizing a total of 1066 individual vibrations. Scaling factors suitable for low-frequency vib...

Density functionals with broad applicability in chemistry.

Abstract: Although density functional theory is widely used in the computational chemistry community, the most popular density functional, B3LYP, has some serious shortcomings: (i) it is better for main-group chemistry than for transition metals; (ii) it systematically underestimates reaction barrier heights; (iii) it is inaccurate for interactions dominated by medium-range correlation energy, such as van der Waals attraction, aromatic−aromatic stacking, and alkane isomerization energies. We have developed a variety of databases for testing and designing new density functionals. We used these data to design new density functionals, called M06-class (and, earlier, M05-class) functionals, for which we enforced some fundamental exact constraints such as the uniform-electron-gas limit and the absence of self-correlation energy. Our M06-class functionals depend on spin-up and spin-down electron densities (i.e., spin densities), spin density gradients, spin kinetic energy densities, and, for nonlocal (also called hybrid)...

A variational eigenvalue solver on a photonic quantum processor

Abstract: Quantum computers promise to efficiently solve important problems that are intractable on a conventional computer. For quantum systems, where the physical dimension grows exponentially, finding the eigenvalues of certain operators is one such intractable problem and remains a fundamental challenge. The quantum phase estimation algorithm efficiently finds the eigenvalue of a given eigenvector but requires fully coherent evolution. Here we present an alternative approach that greatly reduces the requirements for coherent evolution and combine this method with a new approach to state preparation based on ansatze and classical optimization. We implement the algorithm by combining a highly reconfigurable photonic quantum processor with a conventional computer. We experimentally demonstrate the feasibility of this approach with an example from quantum chemistry--calculating the ground-state molecular energy for He-H(+). The proposed approach drastically reduces the coherence time requirements, enhancing the potential of quantum resources available today and in the near future.

Semiempirical hybrid density functional with perturbative second-order correlation

Abstract: A new hybrid density functional for general chemistry applications is proposed. It is based on a mixing of standard generalized gradient approximations GGAs for exchange by Becke B and for correlation by Lee, Yang, and Parr LYP with Hartree-Fock HF exchange and a perturbative second-order correlation part PT2 that is obtained from the Kohn-Sham GGA orbitals and eigenvalues. This virtual orbital-dependent functional contains only two global parameters that describe the mixture of HF and GGA exchange ax and of the PT2 and GGA correlation c, respectively. The parameters are obtained in a least-squares-fit procedure to the G2/97 set of heat of formations. Opposed to conventional hybrid functionals, the optimum ax is found to be quite large 53% with c=27% which at least in part explains the success for many problematic molecular systems compared to conventional approaches. The performance of the new functional termed B2-PLYP is assessed by the G2/97 standard benchmark set, a second test suite of atoms, molecules, and reactions that are considered as electronically very difficult including transition-metal compounds, weakly bonded complexes, and reaction barriers and comparisons with other hybrid functionals of GGA and meta-GGA types. According to many realistic tests, B2-PLYP can be regarded as the best general purpose density functional for molecules e.g., a mean absolute deviation for the two test sets of only 1.8 and 3.2 kcal/mol compared to about 3 and 5 kcal/mol, respectively, for the best other density functionals. Very importantly, also the maximum and minium errors outliers are strongly reduced by about 10‐20 kcal/mol. Furthermore, very good results are obtained for transition state barriers but unlike previous attempts at such a good description, this definitely comes not at the expense of equilibrium properties. Preliminary calculations of the equilibrium bond lengths and harmonic vibrational frequencies for diatomic molecules and transition-metal complexes also show very promising results. The uniformity with which B2-PLYP improves for a wide range of chemical systems emphasizes the need of virtual orbital-dependent terms that describe nonlocal electron correlation in accurate exchange-correlation functionals. From a practical point of view, the new functional seems to be very robust and it is thus suggested as an efficient quantum chemical method of general purpose. © 2006 American Institute of Physics. DOI: 10.1063/1.2148954