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1.
J Chem Theory Comput ; 15(12): 6703-6711, 2019 Dec 10.
Artigo em Inglês | MEDLINE | ID: mdl-31652067

RESUMO

We develop a stochastic resolution of identity approach to the real-time second-order Green's function (real-time sRI-GF2) theory, extending our recent work for imaginary-time Matsubara Green's function [ Takeshita et al. J. Chem. Phys. 2019 , 151 , 044114 ]. The approach provides a framework to obtain the quasi-particle spectra across a wide range of frequencies and predicts ionization potentials and electron affinities. To assess the accuracy of the real-time sRI-GF2, we study a series of molecules and compare our results to experiments as well as to a many-body perturbation approach based on the GW approximation, where we find that the real-time sRI-GF2 is as accurate as self-consistent GW. The stochastic formulation reduces the formal computatinal scaling from O(Ne5) down to O(Ne3) where Ne is the number of electrons. This is illustrated for a chain of hydrogen dimers, where we observe a slightly lower than cubic scaling for systems containing up to Ne ≈ 1000 electrons.

2.
J Chem Phys ; 151(4): 044114, 2019 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-31370516

RESUMO

We develop a stochastic resolution of identity representation to the second-order Matsubara Green's function (sRI-GF2) theory. Using a stochastic resolution of the Coulomb integrals, the second order Born self-energy in GF2 is decoupled and reduced to matrix products/contractions, which reduces the computational cost from O(N5) to O(N3) (with N being the number of atomic orbitals). The current approach can be viewed as an extension to our previous work on stochastic resolution of identity second order Møller-Plesset perturbation theory [T. Y. Takeshita et al., J. Chem. Theory Comput. 13, 4605 (2017)] and offers an alternative to previous stochastic GF2 formulations [D. Neuhauser et al., J. Chem. Theory Comput. 13, 5396 (2017)]. We show that sRI-GF2 recovers the deterministic GF2 results for small systems, is computationally faster than deterministic GF2 for N > 80, and is a practical approach to describe weak correlations in systems with 103 electrons and more.

3.
J Chem Theory Comput ; 14(7): 3504-3511, 2018 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-29771539

RESUMO

Psi4NumPy demonstrates the use of efficient computational kernels from the open-source Psi4 program through the popular NumPy library for linear algebra in Python to facilitate the rapid development of clear, understandable Python computer code for new quantum chemical methods, while maintaining a relatively low execution time. Using these tools, reference implementations have been created for a number of methods, including self-consistent field (SCF), SCF response, many-body perturbation theory, coupled-cluster theory, configuration interaction, and symmetry-adapted perturbation theory. Furthermore, several reference codes have been integrated into Jupyter notebooks, allowing background, underlying theory, and formula information to be associated with the implementation. Psi4NumPy tools and associated reference implementations can lower the barrier for future development of quantum chemistry methods. These implementations also demonstrate the power of the hybrid C++/Python programming approach employed by the Psi4 program.

4.
J Chem Theory Comput ; 13(10): 4605-4610, 2017 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-28914534

RESUMO

A stochastic orbital approach to the resolution of identity (RI) approximation for 4-index electron repulsion integrals (ERIs) is presented. The stochastic RI-ERIs are then applied to second order Møller-Plesset perturbation theory (MP2) utilizing a multiple stochastic orbital approach. The introduction of multiple stochastic orbitals results in an O(NAO3) scaling for both the stochastic RI-ERIs and stochastic RI-MP2, NAO being the number of basis functions. For a range of water clusters we demonstrate that this method exhibits a small prefactor and observed scalings of O(Ne2.4) for total energies and O(Ne3.1) for forces (Ne being the number of correlated electrons), outperforming MP2 for clusters with as few as 21 water molecules.

5.
J Phys Chem A ; 120(48): 9607-9611, 2016 Dec 08.
Artigo em Inglês | MEDLINE | ID: mdl-27933910

RESUMO

In a previous paper in this series, we discussed the formation of recoupled pair bonds in the a4Σ- states of CF and SF in which the recoupling process was essentially complete at the equilibrium geometry of the molecule. In this paper, we examine the a4Σ- state of oxygen monofluoride (OF), which could also have a recoupled pair bond. Unlike the other two molecules, generalized valence bond calculations predict that the recoupling in OF is woefully incomplete at Re and the resulting potential energy curve for the OF(a4Σ-) state is purely repulsive; the binding energy, ≈11 kcal/mol, is entirely due to dynamical correlation. A number of factors account for these differences, but the nature of the dominant correlation effect in the oxygen 2p lone pair as well as the spatial extent of the 2p orbital are paramount.

6.
J Phys Chem A ; 120(34): 6846-50, 2016 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-27501366

RESUMO

Crabtree et al. recently reported the microwave spectrum of nitrosyl-O-hydroxide (trans-NOOH), an isomer of nitrous acid, and found that this molecule has the longest O-O bond ever observed: 1.9149 Å ± 0.0005 Å. This is in marked contrast to the structure of the valence isoelectronic trans-NSOH molecule, which has a normal NS-OH bond length and strength. Generalized valence bond calculations show that the long bond in trans-NOOH is the result of a weak through-pair interaction that singlet couples the spins of the electrons in singly occupied orbitals on the hydroxyl radical and nitrogen atom, an interaction that is enhanced by the intervening lone pair of the oxygen atom in NO. The NS-OH bond is the result of the formation of a stable recoupled pair bond dyad, which accounts for both its length and strength.

7.
J Chem Phys ; 145(4): 044112, 2016 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-27475353

RESUMO

Development of exponentially scaling methods has seen great progress in tackling larger systems than previously thought possible. One such technique, full configuration interaction quantum Monte Carlo, is a useful algorithm that allows exact diagonalization through stochastically sampling determinants. The method derives its utility from the information in the matrix elements of the Hamiltonian, along with a stochastic projected wave function, to find the important parts of Hilbert space. However, the stochastic representation of the wave function is not required to search Hilbert space efficiently, and here we describe a highly efficient deterministic method that can achieve chemical accuracy for a wide range of systems, including the difficult Cr2 molecule. We demonstrate for systems like Cr2 that such calculations can be performed in just a few cpu hours which makes it one of the most efficient and accurate methods that can attain chemical accuracy for strongly correlated systems. In addition our method also allows efficient calculation of excited state energies, which we illustrate with benchmark results for the excited states of C2.

8.
J Phys Chem A ; 120(17): 2720-6, 2016 05 05.
Artigo em Inglês | MEDLINE | ID: mdl-27070292

RESUMO

Ozone (O3) and sulfur dioxide (SO2) are valence isoelectronic species, yet their properties and reactivities differ dramatically. In particular, O3 is highly reactive, whereas SO2 is chemically relatively stable. In this paper, we investigate serial addition of hydrogen atoms to both the terminal atoms of O3 and SO2 and to the central atom of these species. It is well-known that the terminal atoms of O3 are much more amenable to bond formation than those of SO2. We show that the differences in the electronic structure of the π systems in the parent triatomic species account for the differences in the addition of hydrogen atoms to the terminal atoms of O3 and SO2. Further, we find that the π system in SO2, which is a recoupled pair bond dyad, facilitates the addition of hydrogen atoms to the sulfur atom, resulting in stable HSO2 and H2SO2 species.

9.
J Phys Chem A ; 120(11): 1763-78, 2016 Mar 24.
Artigo em Inglês | MEDLINE | ID: mdl-26909685

RESUMO

In this article we describe the unique insights into the electronic structure of molecules provided by generalized valence bond (GVB) theory. We consider selected prototypical hydrocarbons as well as a number of hypervalent molecules and a set of first- and second-row valence isoelectronic species. The GVB wave function is obtained by variationally optimizing the orbitals and spin coupling in the valence bond wave function. The GVB wave function is a generalization of the Hartree-Fock (HF) wave function, lifting the double occupancy restriction on a subset of the HF orbitals as well as the associated orthogonality and spin coupling constraints. The GVB wave function includes a major fraction (if not all) of the nondynamical correlation energy of a molecule. Because of this, GVB theory properly describes bond formation and can answer one of the most compelling questions in chemistry: How are atoms changed by molecular formation? We show that GVB theory provides a unified description of the nature of the bonding in all of the above molecular species as well as contributing new insights into the well-known, but poorly understood, first-row anomaly.

11.
J Phys Chem A ; 119(28): 7683-94, 2015 Jul 16.
Artigo em Inglês | MEDLINE | ID: mdl-26068052

RESUMO

There are many well-known differences in the physical and chemical properties of ozone (O3) and sulfur dioxide (SO2). O3 has longer and weaker bonds than O2, whereas SO2 has shorter and stronger bonds than SO. The O-O2 bond is dramatically weaker than the O-SO bond, and the singlet-triplet gap in SO2 is more than double that in O3. In addition, O3 is a very reactive species, while SO2 is far less so. These disparities have been attributed to variations in the amount of diradical character in the two molecules. In this work, we use generalized valence bond (GVB) theory to characterize the electronic structure of ozone and sulfur dioxide, showing O3 does indeed possess significant diradical character, whereas SO2 is effectively a closed shell molecule. The GVB results provide critical insights into the genesis of the observed difference in these two isoelectronic species. SO2 possesses a recoupled pair bond dyad in the a"(π) system, resulting in SO double bonds. The π system of O3, on the other hand, has a lone pair on the central oxygen atom plus a pair of electrons in orbitals on the terminal oxygen atoms that give rise to a relatively weak π interaction.

12.
J Phys Chem A ; 119(9): 1456-63, 2015 Mar 05.
Artigo em Inglês | MEDLINE | ID: mdl-25627935

RESUMO

The electronic structure of the ground state of NO and those of F(NO) and H(NO), that is, the XNO and NOX isomers with X = F, H, were analyzed within the framework of generalized valence bond theory. In distinct contrast to the ground state of NS, it was found that the two-center, three-electron π interaction in NO(X(2)Π) is composed of a lone pair on O and a singly occupied orbital on N. Thus, F and H addition to NO preferentially leads to FNO and HNO. Somewhat surprisingly, the NOF and NOH isomers were found to be weakly bound, although for different reasons. The NOF state has a very unusual through-pair interaction with a NO-F bond length 0.444 Å longer than its covalent counterpart in OF(X(2)Π), while NOH arises from the N((2)D) + OH(X(2)Π) separated atom limit, similar to what we found for NSH.

13.
J Phys Chem A ; 119(9): 1446-55, 2015 Mar 05.
Artigo em Inglês | MEDLINE | ID: mdl-25628034

RESUMO

The electronic structures of the ground states (X(2)Π) of NS and those (X(1)A') of F(NS) and H(NS), where X(NS) collectively refers to the XNS and NSX isomers, were analyzed within the framework of generalized valence bond theory. The ground state of NS has a recoupled pair π bond, which has a profound effect on its reactivity. For example, the lowest-energy isomer of F(NS) is NSF, which has a recoupled pair bond dyad with N-SF and NS-F bonds lengths and strengths similar to their covalent counterparts in NS and SF. The ground state of NSH, on the other hand, is only weakly bound with a NS-H bond energy 40.20 kcal/mol smaller than that in SH and a N-SH bond energy 40.20 kcal/mol less than that in NS. At its equilibrium geometry, the NSH molecule is best viewed as derived from the N((2)D) + SH(X(2)Π) separated fragments, with the weak NS-H bond resulting from unfavorable interactions between the SH bond pair and the nitrogen lone pair. Addition of F/H atoms to the nitrogen atom in NS disrupts the NS recoupled pair bond, which weakens both the FN-S/HN-S and F-NS/H-NS bonds. In contrast to the formation of recoupled pair σ bonds, formation of the recoupled pair π bond in NS is expressed as a change in the spin-coupling coefficients, rather than an interchange of the orbitals.

14.
J Chem Phys ; 142(3): 034113, 2015 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-25612695

RESUMO

The number of singly occupied orbitals in the ground-state atomic configuration of an element defines its nominal valence. For carbon and sulfur, with two singly occupied orbitals in their (3)P ground states, the nominal valence is two. However, in both cases, it is possible to form more bonds than indicated by the nominal valence--up to four bonds for carbon and six bonds for sulfur. In carbon, the electrons in the 2s lone pair can participate in bonding, and in sulfur the electrons in both the 3p and 3s lone pairs can participate. Carbon 2s and sulfur 3p recoupled pair bonds are the basis for the tetravalence of carbon and sulfur, and 3s recoupled pair bonds enable sulfur to be hexavalent. In this paper, we report generalized valence bond as well as more accurate calculations on the a(4)Σ(-) states of CF and SF, which are archetypal examples of molecules that possess recoupled pair bonds. These calculations provide insights into the fundamental nature of recoupled pair bonds and illustrate the key differences between recoupled pair bonds formed with the 2s lone pair of carbon, as a representative of the early p-block elements, and recoupled pair bonds formed with the 3p lone pair of sulfur, as a representative of the late p-block elements.

15.
J Chem Phys ; 142(3): 034114, 2015 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-25612696

RESUMO

Formation of a bond between a second ligand and a molecule with a recoupled pair bond results in a recoupled pair bond dyad. We examine the recoupled pair bond dyads in the a(3)B1 states of CF2 and SF2, which are formed by the addition of a fluorine atom to the a(4)Σ(-) states of CF and SF, both of which possess recoupled pair bonds. The two dyads are very different. In SF2, the second FS-F bond is very strong (De = 106.3 kcal/mol), the bond length is much shorter than that in the SF(a(4)Σ(-)) state (1.666 Å versus 1.882 Å), and the three atoms are nearly collinear (θe = 162.7°) with only a small barrier to linearity (0.4 kcal/mol). In CF2, the second FC-F bond is also very strong (De = 149.5 kcal/mol), but the bond is only slightly shorter than that in the CF(a(4)Σ(-)) state (1.314 Å versus 1.327 Å), and the molecule is strongly bent (θe = 119.0°) with an 80.5 kcal/mol barrier to linearity. The a(3)B1 states of CF2 and SF2 illustrate the fundamental differences between recoupled pair bond dyads formed from 2s and 3p lone pairs.

16.
J Chem Theory Comput ; 9(10): 4444-52, 2013 Oct 08.
Artigo em Inglês | MEDLINE | ID: mdl-26589162

RESUMO

The ground states (X(2)A″) of HSO and SOH are extremely close in energy, yet their molecular structures differ dramatically, e.g., re(SO) is 1.485 Å in HSO and 1.632 Å in SOH. The SO bond is also much stronger in HSO than in SOH: 100.3 kcal/mol versus 78.8 kcal/mol [RCCSD(T)-F12/AVTZ]. Similar differences are found in the SO2 isomers, SOO and OSO, depending on whether the second oxygen atom binds to oxygen or sulfur. We report generalized valence bond and RCCSD(T)-F12 calculations on HSO/SOH and OSO/SOO and analyze the bonding in all four species. We find that HSO has a shorter and stronger SO bond than SOH due to the presence of a recoupled pair bond in the π(a″) system of HSO. Similarly, the bonding in SOO and OSO differs greatly. SOO is like ozone and has substantial diradical character, while OSO has two recoupled pair π bonds and negligible diradical character. The ability of the sulfur atom to form recoupled pair bonds provides a natural explanation for the dramatic variation in the bonding in these and many other sulfur-oxygen compounds.

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