We present a general implementation of the resolution-of-the-identity (RI) and Cholesky decomposition (CD) representations of electron repulsion integrals within the coupled-cluster with single and double substitutions (CCSD) and equation-of-motion (EOM) family of methods. The CCSD and EOM-CCSD equations are rewritten to eliminate the storage of the largest four-index intermediates leading to a...

Two new schemes for computing molecular total atomization energies (TAEs) and/or heats of formation (∆H • f) of first-and second-row compounds to very high accuracy are presented. The more affordable scheme, W1 (Weizmann-1) theory, yields a mean absolute error of 0.30 kcal/mol and includes only a single, molecule-independent, empirical parameter. It requires CCSD (coupled cluster with all singl...

Methods for fast and reliable computation of electronic excitation energies are in short supply, and little is known about their systematic performance. This work reports a comparison of several low-scaling approximations to the equation of motion coupled cluster singles and doubles (EOM-CCSD) and linear-response coupled cluster singles and doubles (LR-CCSD) equations with other single referenc...

The coupled cluster CCSD~T! method provides a theoretically sound, accurate description of the electronic structure of a wide range of molecules. To obtain accurate results, however, very large basis sets must be used. Since the computational cost of CCSD~T! calculations formally increases with the seventh power of the number of basis functions (N), the CCSD~T! method can only be applied to a r...

While CCSD(T) with spin-restricted Hartree–Fock (RHF) orbitals has long been lauded for its ability to accurately describe closed-shell interactions, the performance of on open-shell species is much more erratic, especially when using a spin-unrestricted HF (UHF) reference. Previous studies have shown improved treatment systems non-HF set molecular orbitals, like Brueckner or Kohn–Sham density ...

Several approximate correlation methods have been assessed for bond breaking reactions in BH, HF, and CH4 by comparison to the full configuration interaction limit. Second-order Møller–Plesset perturbation theory, coupled-cluster singles and doubles ~CCSD!, coupled-cluster with perturbative triples @CCSD~T!#, and the hybrid density-functional method Becke three parameter Lee–Yang– Parr have bee...

Moller-Plesset second-order (MP2) perturbation theory breaks down at molecular geometries which are far away from equilibrium. We decompose the MP2 energy into contributions from different orbital subspaces and show that the divergent behavior of the MP2 energy comes from the excitations located within a small (or sometimes even the minimal) active space. The divergent behavior of the MP2 energ...

It is typically assumed that once a Hartree-Fock (HF) reference wave function is determined, the correlated wave function obtained from that HF wave function describes the same electronic state. In this paper, we report the appearance of multiple CCSD solutions obtained from the UHF reference wave function for the known ground state of a chemically interesting molecule, NiH. To determine a corr...

The importance of post-CCSD(T) corrections as high CCSDTQ56 for ground-state spectroscopic constants ($D_e$, $\omega_e$, $\omega_ex_e$, and $\alpha_e$) has been surveyed a sample two dozen mostly heavy-atom diatomics spanning broad range static correlation strength. While CCSD(T) is known to be an unusually felicitous `Pauling point' between accuracy computational cost, performance leaves somet...

The reliability of density functional theory and other electronic structure methods is examined for anharmonicities and spectroscopic constants of the ground electronic states of several diatomic molecules. The equilibrium bond length re , harmonic vibrational frequency ve , vibrational anharmonicity vexe , rotational constant Be , centrifugal distortion constant D̄e , and vibration-rotation int...