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.

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

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...

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

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)...

Density functionals with broad applicability in chemistry.

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.

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A variational eigenvalue solver on a photonic quantum processor

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

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Semiempirical hybrid density functional with perturbative second-order correlation

The B3LYP, BLYP, and BP86 variants of density functional theory, in conjunction with double and triple ζ basis sets, have been examined for SN2 reactions of the type CH3X + F- → CH3F + X- (X = F, C...

Assessment of Density Functional Theory for Model SN2 Reactions: CH3X + F- (X = F, Cl, CN, OH, SH, NH2, PH2)

High-level ab initio quantum chemical studies have been performed to ascertain the equilibrium molecular structure and ring-puckering inversion barrier of cyclopentene, as well as the complete infrared and Raman spectra of cyclopentene-d 0 , -1-d 1 , -1,2,3,3-d 4 , and -d 8 . The planar (C 2) and puckered (C s ) structures of cyclopentene have been optimized at the SCF and MP2 levels, and reasonable agreement with available experimental data is found

The puckering inversion barrier and vibrational spectrum of cyclopentene. A scaled quantum mechanical force field algorithm

A comprehensive anharmonic vibrational analysis of isotopic ketenes has been performed on the basis of a complete ab initio quartic force field constructed by means of second‐order Mo/ller–Plesset perturbation theory (MP2) and the coupled‐cluster singles and doubles (CCSD) approach, augmented for structural optimizations by a contribution for connected triple excitations [CCSD(T)]. The atomic‐orbital basis sets of the study entailed C,O(10s6p/5s4p) and H(6s/4s) spaces multiply polarized in the valence region to give QZ(2d,2p) and QZ(2d1f,2p1d) sets. An iterative anharmonic vibrational refinement of a limited set of quadratic scaling parameters on 27 fundamentals of H2CCO, HDCCO, D2CCO, and H2C13CO generates a final quartic force field which reproduces the empirical νi data with an average absolute error of only 1.1 cm−1. This force field yields a complete and self‐consistent set of Coriolis (ζij), vibrational anharmonic (χij), vibration–rotation interaction (αi), and quartic and sextic centrifugal distort...

The anharmonic force field and equilibrium molecular structure of ketene

This article describes a detailed theoretical analysis of the reaction between allyl and propargyl. In this analysis, we employ high-level electronic structure calculations to characterize the potential energy surface and various forms of transition-state theory (TST) to calculate microcanonical, J-resolved rate coefficients-conventional TST for isomerizations, and the variable reaction coordinate form of variational TST for the “barrierless” association/dissociation processes. These rate coefficients are used in a time-dependent, multiple-well master equation to determine phenomenological rate coefficients, k(T,p), for various product channels. The analysis indicates that the formation of (cyclic) c-C6H7 and c-C6H8 species is suppressed by elevated pressure. Overall, the results suggest that the formation of these five-membered rings from the reaction is not as important as previously thought. A simplified description of the kinetics of the reaction is discussed, and corresponding rate coefficients are p...

Reactions between resonance-stabilized radicals: propargyl + allyl.

A collection of recent state-of-the-art ab initio results is presefted pertaining to anharmonic vibrational analyses of non-rigid molecules. To demonstrate the performance of electronic structure theory in predicting features of potential energy surfaces to quantitative accuracy, investigations of energy barriers for large-amplitude vibrations in ammonia, isocyanic acid, ethane, and cyclopentene are reported. Anharmonic vibrational analyses based on self-consistent-field and configuration interaction quartic force fields are critically evaluated by focusing on representative results for hydrogen sulfide and nitrous oxide. The use of nonstationary reference structures to improve predictions of anharmonic force fields is reviewed and documented. A summary of an investigation using the complete state-of-the-art armamentarium to elucidate vibrational anharmonicity, vibration-rotation interaction, and the precise r e structure of the quasilinear HNCO molecule is given. Finally, difficulties arising in the application of nonstationary reference geometries in vibrational analyses of large-amplitude motions are highlighted to avert potential pitfalls in future ab initio studies.

Wesley D. Allen

Papers

Assessment of Density Functional Theory for Model SN2 Reactions: CH3X + F- (X = F, Cl, CN, OH, SH, NH2, PH2)

The puckering inversion barrier and vibrational spectrum of cyclopentene. A scaled quantum mechanical force field algorithm

The anharmonic force field and equilibrium molecular structure of ketene

Reactions between resonance-stabilized radicals: propargyl + allyl.

Ab Initio Anharmonic Vibrational Analyses of Non-Rigid Molecules