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

Kumada catalyst-transfer polycondensation (KCTP) is an effective method for the controlled polymerization of conjugated polymers. Nevertheless, side reactions leading to early termination and unwanted chain coupling cause deviations from the target molecular weight, along with increasing polydispersity and end group variation. The departure from the KCTP cycle stems from a disproportionation reaction that leads to experimentally observed side products. The disproportionation energies for a series of nickel-based initiators containing bidentate phosphino attendant ligands were computed using density functional theory at the B3LYP/DZP level. The initiator was found to be less favorable toward disproportionation by 0.5 kcal mol–1 when ligated by 1,3-bis(diphenylphosphino)propane (dppp) rather than 1,2-bis(diphenylphosphino)ethane (dppe). Trends in disproportionation energy (Edisp) with a variety of bidentate phosphine ligands match experimental observations of decreased polymerization control. Theoretical Ed...

On the Role of Disproportionation Energy in Kumada Catalyst-Transfer Polycondensation

https://pubs.rsc.org/en/Content/ArticlePDF/2016/CC/C6CC01756H

Tunnelling in carbonic acid

This paper reports three independent studies In the first study, the infrared band shapes and relative intensities of gaseous thiirane‐d4 (ethylene sulfide‐d4, C2D4S), the Raman spectrum of liquid thiirane‐d4, and infrared spectra of gaseous cis‐ and trans‐1, 2‐dideuteriothiirane, (CHD)2S, are reported for the first time The vibrational spectra of C2H4S, C2D4S, and some bands of cis‐(CHD)2S are assigned from the symmetry analysis, group frequencies, infrared band shapes, and Raman polarization data The frequencies so assigned are used to derive a modified valence force field, (MVFF), which reproduces them well, allows the remaining fundamental frequencies of cis‐(CHD)2S to be found, and allows the spectrum of trans‐(CHD)2S to be assigned The MVFF is then further refined to optimize the fit to the 46 assigned frequencies of the four molecules Twenty four nonzero force constants fit the 46 frequencies with an average error of 04% The assignment is thus well based and self‐consistent Inthe second stu

The experimental vibrational spectra, vibrational assignment, and normal coordinate analysis of thiirane‐h4 and ‐d4 and cis‐ and trans‐1,2‐dideuteriothiirane: Ab initio theoretical IR spectra of thiirane, thiirene, and isotopically substituted derivatives

It has been suggested [F. H. Stillinger, J. Chem. Phys. 112, 9711 (2000)] that the convergence or divergence of Moller-Plesset perturbation theory is determined by a critical point at a negative value of the perturbation parameter z at which an electron cluster dissociates from the nuclei. This conjecture is examined using configuration-interaction computations as a function of z and using a quadratic approximant analysis of the high-order perturbation series. Results are presented for the He, Ne, and Ar atoms and the hydrogen fluoride molecule. The original theoretical analysis used the true Hamiltonian without the approximation of a finite basis set. In practice, the singularity structure depends strongly on the choice of basis set. Standard basis sets cannot model dissociation to an electron cluster, but if the basis includes diffuse functions then it can model another critical point corresponding to complete dissociation of all the valence electrons. This point is farther from the origin of the z plane than is the critical point for the electron cluster, but it is still close enough to cause divergence of the perturbation series. For the hydrogen fluoride molecule a critical point is present even without diffuse functions. The basis functions centered on the H atom are far enough from the F atom to model the escape of electrons away from the fluorine end of the molecule. For the Ar atom a critical point for a one-electron ionization, which was not previously predicted, seems to be present at a positive value of the perturbation parameter. Implications of the existence of critical points for quantum-chemical applications are discussed.

On the nature of the Møller-Plesset critical point.

A global six-dimensional potential surface for the hydridotrioxygen radical (HOOO) is needed for an accurate assessment of its atmospheric abundance. We report inertial dipole moment components obtained from Stark spectra of the trans-HOOO system solvated in superfluid helium, and these are shown to be stringent benchmarks for theoretical computations of the potential surface. Computed dipole moment components at the CCSD(T)/CBS equilibrium geometry disagree qualitatively with the experimental values. The role of large-amplitude motion and vibrational averaging is assessed by computing the ground-state wave function on a relaxed, two-dimensional potential surface for the HO1O2O3 torsional and O1O2 bond-stretching coordinates. The experimental and computed vibrationally averaged dipole moments agree only after shifting the potential along the O1O2 bond coordinate, indicating that single-reference CCSD(T)/CBS computations underestimate re(O1O2) by ∼0.08 A. An optimized trans-HOOO geometry at the composite a...

Wesley D. Allen

Papers

On the Role of Disproportionation Energy in Kumada Catalyst-Transfer Polycondensation

Tunnelling in carbonic acid

The experimental vibrational spectra, vibrational assignment, and normal coordinate analysis of thiirane‐h4 and ‐d4 and cis‐ and trans‐1,2‐dideuteriothiirane: Ab initio theoretical IR spectra of thiirane, thiirene, and isotopically substituted derivatives

On the nature of the Møller-Plesset critical point.

Dipole Moment of the HOOO Radical: Resolution of a Structural Enigma