scispace - formally typeset
Search or ask a question

Showing papers on "Coupled cluster published in 1998"


Journal ArticleDOI
TL;DR: In this article, the correlation-consistent polarized valence and core-valence basis sets up to sextuple zeta quality are employed to compute the correlation energies of Ne, N2, and H2O at fixed experimental geometries at the levels of second-order perturbation theory (MP2) and coupled cluster theory with singles and doubles excitations (CCSD) with a perturbative triples correction (T)).

1,961 citations


Journal ArticleDOI
TL;DR: The convergence of ab initio predictions to the one-and n-particle limits has been systematically explored for several conformational energy prototypes as mentioned in this paper, including the inversion barriers of ammonia, water, and isocyanic acid, the torsional barrier of ethane, and the E/Z rotamer separation of formic acid.
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...

644 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present new diagnostics for coupled-cluster and Moller-Plesset perturbation theory, readily computed from the single substitution amplitudes in the coupledcluster singles and doubles wave function or in the second-order Moller Plesset wave function.

261 citations


Journal ArticleDOI
TL;DR: In this paper, the authors exploit the systematic convergence properties of the correlation consistent family of basis sets, and estimate the complete basis set limits for a large number of the Gaussian-2 energetic properties.
Abstract: The Gaussian-2 (G2) collection of atoms and molecules has been studied with Hartree–Fock and correlated levels of theory, ranging from second-order perturbation theory to coupled cluster theory with noniterative inclusion of triple excitations. By exploiting the systematic convergence properties of the correlation consistent family of basis sets, complete basis set limits were estimated for a large number of the G2 energetic properties. Deviations with respect to experimentally derived energy differences corresponding to rigid molecules were obtained for 15 basis set/method combinations, as well as the estimated complete basis set limit. The latter values are necessary for establishing the intrinsic error for each method. In order to perform this analysis, the information generated in the present study was combined with the results of many previous benchmark studies in an electronic database, where it is available for use by other software tools. Such tools can assist users of electronic structure codes i...

243 citations


Journal ArticleDOI
TL;DR: In this paper, a hierarchy of feasible approximations to the nondynamical correlation energy based on coupled-cluster theory with variationally optimized orbitals is introduced, denoted as VOO-CCD, or VOD.
Abstract: The nondynamical correlation energy may be defined as the difference between full configuration interaction within the space of all valence orbitals and a single determinant of molecular orbitals (Hartree–Fock theory). In order to describe bond breaking, diradicals, and other electronic structure problems where Hartree–Fock theory fails, a reliable description of nondynamical correlation is essential as a starting point. Unfortunately, the exact calculation of nondynamical correlation energy, as defined above, involves computational complexity that grows exponentially with molecular size and is thus unfeasible beyond systems of just two or three heavy atoms. We introduce a new hierarchy of feasible approximations to the nondynamical correlation energy based on coupled-cluster theory with variationally optimized orbitals. The simplest member of this hierarchy involves connected double excitations within the variationally optimized valence active space and may be denoted as VOO-CCD, or VOD. VOO-CCD is size-consistent, has computational complexity proportional to the sixth power of molecule size, and is expected to accurately approximate the nondynamical correlation energy in such cases as single bond dissociation, diradicals, and anti-ferromagnetic coupling. We report details of our implementation of VOO-CCD and illustrate that it does indeed accurately recover the nondynamical correlation energy for challenging multireference problems such as the torsion of ethylene and chemical bond breaking.

240 citations


Journal ArticleDOI
TL;DR: In this article, an atomic integral-direct implementation of molecular linear response properties and excited-state one-electron properties is presented for the coupled cluster models CCS, CC2, and CCSD.
Abstract: An atomic integral-direct implementation of molecular linear-response properties and excited-state one-electron properties is presented for the coupled cluster models CCS, CC2, and CCSD. Sample calculations are presented for the polarizability of N2 and for excited-state one-electron properties and transition-properties of furan.

198 citations


Journal ArticleDOI
TL;DR: A state-specific coupled cluster method based on a reference function composed of determinants spanning a complete active space (CAS) that is expected to provide a uniform description over a wide range of molecular geometries.
Abstract: We present in this paper a state-specific coupled cluster method based on a reference function composed of determinants spanning a complete active space (CAS). The method treats all the reference d...

195 citations


Journal ArticleDOI
TL;DR: In this paper, perturbation corrections through fifth order in the many-body perturbations theory energy with respect to a coupled cluster singles and doubles reference have been derived and analyzed, and the proposed corrections have been applied to several small molecules to test their performance compared to full configuration interaction.
Abstract: Perturbation corrections through fifth order in the many-body perturbation theory energy with respect to a coupled cluster singles and doubles reference have been derived and analyzed. The formulas employ the T1 and T2 amplitudes obtained as a solution of the coupled cluster singles and doubles equations. Four different energy functionals have been considered as a starting point in the derivation: the regular coupled cluster energy expression, the coupled cluster functional incorporating Λ amplitudes, the one constructed via an expectation value coupled cluster method, and that obtained on the basis of the extended coupled cluster method. The proposed corrections have been applied to several small molecules to test their performance compared to full configuration interaction. The fourth-order Λ-based formulas improve upon CCSD(T), (coupled cluster singles and doubles with noniterative triples), while the best fifth-order formulas reduce the fourth-order error by about two-thirds. We also introduce a facto...

189 citations


Book ChapterDOI
TL;DR: In this article, two alternative paradigms for multi-reference coupled cluster (MRCC) are discussed, the decontracted description and the contracted description, where the combining coefficients appearing in the reference function have to remain frozen in the equations for the cluster amplitudes.
Abstract: The traditional multi-reference coupled cluster (MRCC) methods are based on effective hamiltonian formalism and often suffer from the problem of intruders. A state-specific MRCC approach, focusing on only one state, offers the attractive possibility of avoiding intruders while at the same time incorporating the nondynamical correlation in a size-extensive manner. In this paper we discuss two alternative paradigms which allow us to achieve this goal. The first, to be called the decontracted description, we deliberately retain certain linearly dependent cluster amplitudes and allow the combining coefficients of the reference determinants to be updated to their values for the exact function. The presence of the linearly dependent cluster amplitudes requires imposition of suitable sufficiency conditions, which are invoked in a manner which naturally ensures size-extensivity. In the second approach, to be called the contracted description, we would generate a cluster expansion with respect to the entire reference function consisting of a combination of reference determinants and retain only the linearly independent cluster-amplitudes in the cluster expansion. For an efficient implementation of the formalism, we shall introduce the notion of extended normal ordering and an analogue of Wick's theorem which uses the entire reference function as the multi-determinantal analogue of the vacuum. This necessarily imposes the restriction that the combining coefficients appearing in the reference function have to remain frozen in the equations for the cluster amplitudes. Relaxation of the coefficients can be acheived only after the cluster-amplitudes with the current coefficients are solved.

181 citations


Journal ArticleDOI
TL;DR: In this article, the frequency-dependent first (β) and second (γ) hyperpolarizabilities for the set of small molecules, N2, CO2, CS2, C2H4, NH3, CO, HF, H2O, and CH4, were compared to Hartree-Fock and correlated ab initio calculations, as well as to experimental results.
Abstract: In this paper we present time-dependent density functional calculations on frequency-dependent first (β) and second (γ) hyperpolarizabilities for the set of small molecules, N2, CO2, CS2, C2H4, NH3, CO, HF, H2O, and CH4, and compare them to Hartree–Fock and correlated ab initio calculations, as well as to experimental results. Both the static hyperpolarizabilities and the frequency dispersion are studied. Three approximations to the exchange-correlation (xc) potential are used: the widely used Local Density Approximation (LDA), the Becke–Lee–Yang–Parr (BLYP) Generalized Gradient Approximation (GGA), as well as the asymptotically correct Van Leeuwen–Baerends (LB94) potential. For the functional derivatives of the xc potential the Adiabatic Local Density Approximation (ALDA) is used. We have attempted to estimate the intrinsic quality of these methods by using large basis sets, augmented with several diffuse functions, yielding good agreement with recent numerical static LDA results. Contrary to claims which have appeared in the literature on the basis of smaller studies involving basis sets of lesser quality, we find that the static LDA results for β and γ are severely overestimated, and do not improve upon the (underestimated) Hartree–Fock results. No improvement is provided by the BLYP potential which suffers from the same incorrect asymptotic behavior as the LDA potential. The results are however clearly improved upon by the LB94 potential, which leads to underestimated results, slightly improving the Hartree–Fock results. The LDA and BLYP potentials overestimate the frequency dependence as well, which is once again improved by the LB94 potential. Future improvements are expected to come from improved models for asymptotically correct exchange-correlation potentials. Apart from the LB94 potential used in this work, several other asymptotically correct potentials have recently been suggested in the literature and can also be expected to improve considerably upon the relatively poor LDA and GGA results, for both the static properties and their frequency dependence.

165 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of core correlation has been accounted for, and the basis set extension effects of diffuse functions and inner polarization functions appear to be nearly perfectly additive, which is the best known result.
Abstract: The total atomization energies, geometries, and anharmonic force fields of the SO and SO2 molecules have been studied at the augmented coupled cluster [CCSD(T)] level near the one-particle basis set limit. The effect of core correlation has been accounted for. The addition of high-exponent d and f “inner polarization functions” to the sulfur basis set was found to be essential for obtaining reliable molecular geometries. The differential effect of core correlation on computed properties is in fact much less important. The CCSD(T) one-particle basis set limit for the symmetric stretching frequencies appears to be higher than the exact value, while the antisymmetric stretching frequency benefits from an error cancellation. The basis set extension effects of diffuse functions and inner polarization functions appear to be nearly perfectly additive. Our best computed atomization energies and geometries agree to within 0.1 kcal/mol, 0.0004 A, and 0.03 degrees with experiment: The errors in the fundamentals of SO2 are +3.9, −0.4, and +0.4 cm−1. The best computed quartic force field for SO2 should provide a good starting point for a new experimental refinement.

Journal ArticleDOI
TL;DR: The reduced multireference (RMR) coupled cluster method restricted to singly and doubly excited clusters (CCSD) as discussed by the authors is employed to compute potential energy surfaces for the HF, F2 and H2O molecules over a wide range of geometries using basis sets of a double zeta (DZ) and DZ plus polarization quality.
Abstract: The so-called reduced multireference (RMR) coupled cluster method restricted to singly and doubly excited clusters (CCSD) [see X. Li and J. Paldus, J. Chem. Phys. 107, 6257 (1997)] is employed to compute potential energy surfaces for the HF, F2 and H2O molecules over a wide range of geometries using basis sets of a double zeta (DZ) and DZ plus polarization (DZP) quality. The RMR-CCSD method belongs to a class of externally corrected CCSD approaches, which rely on a suitable non-CC wave function that is flexible enough to describe the dissociation process at hand and is used as a source of 3- and 4-body cluster amplitudes. These amplitudes are in turn used to achieve a more appropriate decoupling of the full CC chain of equations than that leading to the standard CCSD equations. The RMR-CCSD method employs for this purpose a MR-CISD wave function obtained with a relatively small active or model space. To illustrate the capabilities of this approach, the computed potential energy curves for the HF, F2 and H...

Journal ArticleDOI
TL;DR: In this paper, the tensor approach is applied to formulate theories of electron correlation in nonorthogonal basis sets, and the resulting equations are manifestly invariant to non-orthogonally basis transformations.
Abstract: We apply tensor methods to formulate theories of electron correlation in nonorthogonal basis sets. The resulting equations are manifestly invariant to nonorthogonal basis transformations, between functions spanning either the occupied or virtual subspaces of the one-particle Hilbert space. The tensor approach is readily employed in either first or second quantization. As examples, second-order Mo/ller–Plesset perturbation theory, and coupled cluster theory with single and double substitutions, including noniterative triples, are recast using the tensor formalism. This gives equations which are invariant to larger classes of transformations than existing expressions. Procedures for truncating these equations are discussed.

Book ChapterDOI
TL;DR: In this paper, Hartree et al. extended the Brillouin-Wigner coupled-cluster theory to a multireference case using the Hilbert space approach and presented an explicit form for cluster amplitudes in a spin-orbital form within the CCSD approximation.
Abstract: Recently developed Brillouin-Wigner coupled-cluster theory [I Hubac and P Neogrady, Phys Rev A 50 , 4558 (1994)] is extended to a multireference case using the Hilbert space approach We formulate the so-called single-root (one-state or state-specific) version which deals with one state at a time while employing a multiconfigurational reference wave function Employing the Hilbert space approach to the wave operator, we present an explicit form for cluster amplitudes in a spin-orbital form within the CCSD approximation; ie coupled-cluster method truncated at the single and double excitation level The method is applied to a trapezoidal H4 model system with the use of a two-determinant reference space and the results are compared with the full configuration interaction as well as other correlated multireference techniques The method provides a balanced description of the ground state in both quasidegenerate and nondegenerate regions and deviations from the full configuration interaction energies do not exceed 06 mHartree

Journal ArticleDOI
TL;DR: In this article, a factorization approximation of the T4 operator was proposed, which requires only an n7 procedure and provides results nearly identical to those obtained with the CCSDTQ-1 method.
Abstract: The general inclusion of the T4 operator into the coupled cluster equations requires an n10 computational procedure, and even n9 in the lowest order, as in the CCSDTQ-1 (coupled cluster singles, doubles, triples and lowest-order quadruples) method. That level of n-dependence makes it difficult to apply the method to larger systems. In this paper we circumvent this difficulty by a factorization approximation that requires only an n7 procedure, but that provides results nearly identical to those obtained with the CCSDTQ-1 method. This observation offers a practical and accurate method to go beyond the CCSDT (coupled cluster singles, doubles and triples) approach. We also consider noniterative CCSDT(Qf) (coupled cluster singles, doubles, triples and noniterative quadruples) and CCSD(TQf) (coupled cluster singles and doubles with noniterative triples and quadruples) methods.

Journal ArticleDOI
TL;DR: In this article, the intermediate Hamiltonian technique is applied to reformulate the Fock-space coupled-cluster (FSCC) method with singles and doubles, and a version of FSCC designated to direct calculation of excitation energies is considered.
Abstract: The intermediate Hamiltonian technique is applied to reformulate the Fock-space coupled-cluster (FSCC) method with singles and doubles. A version of FSCC designated to direct calculation of excitation energies is considered. The presented scheme offers several advantages over those used so far like simplification of the equations, smooth convergence, and the possibility of one-root calculation. Results of some illustrative numerical examples are also shown where comparison with other methods as well as with the full configuration interaction results is available.

Journal ArticleDOI
TL;DR: The size-consistent selfconsistent (SC) 2 method as discussed by the authors is based on intermediate Hamiltonians and ensures size-extensivity of any configuration interaction by correcting its diagonal elements.

Journal ArticleDOI
TL;DR: In this article, the authors used integral-direct coupled cluster (ICCSD) to estimate the CCSD(T) dissociation energy of the benzene-argon van der Waals complex.
Abstract: Equilibrium dissociation energies De of the benzene-argon van der Waals complex are calculated in the ground state S0 and in the excited state S1 using integral-direct coupled cluster methods. The results confirm previous investigations of S0, showing that high quality correlation consistent basis sets and connected triple excitations are imperative for a good description of the van der Waals complex. We estimate the CCSD(T) dissociation energy De=389±2 cm-1 for the ground state S0. Using the CCSD linear response approach the frequency shift (redshift) δve=19 cm-1 is obtained. Accurate spectroscopic structural data and frequency shifts δv0 for the 60 1 band of the S1←S0 transition are available for most of the benzene-rare gas atom complexes. However, the experimental determination of absolute dissociation energies of these complexes is connected with much larger uncertainties. The theoretical result agrees very well with the experimentally available redshift, showing that integral-direct coupled cluster methods will become an important tool in the study of van der Waals complexes in the future. © 1998 American Institute of Physics.

Journal ArticleDOI
TL;DR: In this article, the second-order dispersion interaction between the loosely bound electron and the electrons of the neutral host should be included into physical models of dipole-bound anions, and a slow convergence of the Moller−Plesset series for electron binding energies is documented.
Abstract: Dipole-bound anionic states of HCN, (HF)2, CH3CN, C3H2, C4H2, C5H2, and stretched CH3F are studied using extended one-electron basis sets at the coupled cluster level of theory with single, double, and noniterative triple excitations (CCSD(T)). Orbital relaxation and electron correlation corrections to the Koopmans' theorem prediction of electron binding energy are analyzed, and a physical interpretation of low-order corrections is proposed. It is demonstrated that the second-order dispersion interaction between the loosely bound electron and the electrons of the neutral host should be included into physical models of dipole-bound anions. Higher-order electron correlation corrections are also found to be important, and a slow convergence of the Moller−Plesset series for electron binding energies is documented. Modifications of the potential energy surfaces of the above polar molecules upon electron attachment are studied at the second-order Moller−Plesset level, and Franck−Condon factors for the anion/neu...


Journal ArticleDOI
TL;DR: In this article, a quantum cluster equilibrium (QCE) model is proposed for calculating equilibrium properties of liquids by extending the standard quantum statistical thermodynamic treatment of chemical equilibria to the analogous equilibra between molecular clusters, as characterized by modern ab initio techniques.
Abstract: We describe a method for calculating equilibrium properties of liquids by extending the standard quantum statistical thermodynamic treatment of chemical equilibria to the analogous equilibria between molecular clusters, as characterized by modern ab initio techniques. We review the equations of quantum statistical thermodynamics in the canonical ensemble for the case of coupled cluster equilibria, and show how standard treatments of translational and electronic partition functions can be modified to account for excluded-volume and cluster–cluster interaction effects at finite densities. The resulting quantum cluster equilibrium (QCE) model is implemented in a computer program that accepts ab initio input cluster properties and calculates the cluster populations for distinct distributions (phases) satisfying the equilibrium conditions at chosen T, P. We sketch the basic equations and numerical algorithms of the QCE program for neat liquids as well as more general multi-component solution equilibria. The companion paper describes general numerical characteristics of the model, including dependencies on program parameters and cluster input.

Journal ArticleDOI
TL;DR: In this article, the quartic force field of acetylene was determined using the coupled cluster method with all single and double substitutions and quasiperturbative inclusion of connected triple excitations.
Abstract: The quartic force field of acetylene was determined using the CCSD(T) method (coupled cluster with all single and double substitutions and quasiperturbative inclusion of connected triple excitations) with a variety of one-particle basis sets of the atomic natural orbital, correlation consistent, and augmented correlation consistent types. The harmonic πg bending frequency ω4 and the corresponding anharmonicity ω4−ν4 are both found to be extremely sensitive to the basis set used, in particular to the presence of a sufficient complement of diffuse functions. (Due to symmetry cancellation, the corresponding effect on the πu mode, i.e., ω5 and ω5−ν5, is much weaker.) Similar phenomena are observed more generally in bending modes for molecules that possess carbon–carbon multiple bonds. Tentative explanations are advanced. Our best computed quartic force field, which combines CCSD(T)/[6s5p4d3f2g/4s3p2d1f] anharmonicities with a geometry and harmonic frequencies that additionally include inner-shell correlation effects, reproduces the observed fundamentals for HCCH, HCCD, DCCD, H13CCH, and H13C13CH with a mean absolute error of 1.3 cm−1, and the equilibrium rotational constant to four decimal places, without any empirical adjustment. Anharmonicity and quartic resonance constants are in excellent agreement with the recent determination of Temsamani and Herman [J. Chem. Phys. 103, 6371 (1995)], except for the vibrational l-doubling constant R45, for which an adjustment to the computed force field is proposed.

Journal ArticleDOI
TL;DR: In this paper, a single-root multireference Brillouin-Wigner coupled-cluster (MR BWCC) theory is applied to the ground state of the F2 molecule using a two-determinant reference space at the level of the CCSD approximation.
Abstract: Recently developed single-root multireference Brillouin-Wigner coupled-cluster (MR BWCC) theory, which deals with one state at a time while employing a multiconfigurational reference wave function, is applied to the ground state of the F2 molecule using a two-determinant reference space at the level of the CCSD approximation. The method represents a brand-new coupled-cluster (CC) approach to quasidegenerate problems which combines merits of two theories: the single-reference CC method in a nondegenerate case and the Hilbert space MR CC method in quasidegenerate case. The method is able to switch itself from a nondegenerate to a fully degenerate case in a continuous manner, providing thus smooth potential energy surfaces. Moreover, in contrast to the Hilbert space MR CC approaches, it does not contain the so-called coupling terms and completely reduces to the standard single-reference CC method in a highly nondegenerate region. Using a [4s,3p,1d] and [4s,3p,2d,1f ] basis sets, the calculated potential ener...

Journal ArticleDOI
TL;DR: Intermolecular interaction potentials of methane and ethylene dimers were calculated by Hartree-Fock, Moller-Plesset, coupled cluster and density functional methods as mentioned in this paper.

Journal ArticleDOI
TL;DR: In this article, the coupled cluster cubic response function is derived using a time-averaged quasienergy Lagrangian and Fourier component variational perturbation theory, and the results are in excellent agreement with recent experiments for electric field induced second harmonic generation.

Journal ArticleDOI
TL;DR: In this paper, the H5O2+ system has been studied using a variety of coupled cluster methods based on a Brueckner reference determinant with levels of correlation up to double and perturbatively treated connected triple excitations.
Abstract: The H5O2+ system has been studied using a variety of coupled cluster methods based on a Brueckner reference determinant with levels of correlation up to double and perturbatively treated connected triple excitations [B–CCD(T)]. Basis sets as large as the triple-ζ plus double polarization basis augmented with f functions on oxygen and d functions on hydrogen [TZ2P(f,d)] were used. Harmonic vibrational frequencies were also predicted. In contrast with previous high-level ab initio studies, a stationary point of C1 symmetry was not found. An absence of imaginary vibrational frequencies at all levels of theory for the stationary point of C2 symmetry proves it to be the global minimum, lying only ∼0.4 kcal/mol lower in energy than the transition state of Cs symmetry.

Journal ArticleDOI
TL;DR: In this article, the optimized structures and harmonic frequencies for the transition states and intermediates on the ground state potential energy surfaces of ethylenes, including C2H4, C2D4, D2CCH2, and cis-and trans-HDCCDH, related to the molecular and atomic hydrogen elimination channels of photodissociation in VUV were characterized at the B3LYP/6-311G(d,p) level.
Abstract: The optimized structures and harmonic frequencies for the transition states and intermediates on the ground state potential energy surfaces of ethylenes, including C2H4, C2D4, D2CCH2, and cis- and trans-HDCCDH, related to the molecular and atomic hydrogen elimination channels of photodissociation in VUV were characterized at the B3LYP/6-311G(d,p) level. The coupled cluster method, CCSD(T)/6-311+G(3df,2p), was employed to calculate the corresponding energies with the zero-point energy corrections by the B3LYP/6-311G(d,p) approach. Ethylidene was found to be an intermediate in the 1,2-H2 elimination channel. The barrier for the 1,1-H2 elimination was computed to be the lowest (4.10–4.16 eV), while the 1,2-H2 elimination and H loss channels have barriers of a similar height (4.70–4.80 eV). The rate constant for each elementary step of ethylene photodissociation at 193 and 157 nm was calculated according to the RRKM theory based on the ab initio surfaces. The rate equations were subsequently solved, and thus ...

Journal ArticleDOI
TL;DR: Similarity transformed equation-of-motion coupled-cluster calculations on the excited states of free base porphin are reported in this article, and the first calculations to use polarization functions for all of the excited state, and the polarization functions are found to be very important for accurately describing the optically allowed states.
Abstract: Similarity transformed equation-of-motion coupled-cluster calculations on the excited states of free base porphin are reported. These are the first calculations to use polarization functions for all of the excited states, and the polarization functions are found to be very important for accurately describing the optically allowed states. These calculations strongly support the traditional interpretation of the electronic spectrum of free base porphin.

Journal ArticleDOI
TL;DR: In this paper, the ground-state energy and sublattice magnetization of both the square-and triangular-lattices of the Heisenberg antiferromagnets were investigated.
Abstract: We illustrate how the systematic inclusion of multi-spin correlations of the quantum spin–lattice systems can be efficiently implemented within the framework of the coupled-cluster method by examining the ground-state properties of both the square-lattice and the frustrated triangular-lattice quantum antiferromagnets. The ground-state energy and the sublattice magnetization are calculated for the square-lattice and triangular-lattice Heisenberg antiferromagnets, and our best estimates give values for the sublattice magnetization which are 62% and 51% of the classical results for the square and triangular lattices, respectively. We furthermore make a conjecture as to why previous series expansion calculations have not indicated Neel-like long-range order for the triangular-lattice Heisenberg antiferromagnet. We investigate the critical behavior of the anisotropic systems by obtaining approximate values for the positions of phase transition points.

Journal ArticleDOI
TL;DR: In this paper, the reduced multireference (RMR) coupled cluster method with singles and doubles is applied to the dissociation of the ground state of the nitrogen molecule, and it is shown that even with a relatively modest highly truncated reference space one obtains an accurate potential over a wide range of internuclear separations.