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The high-accuracy W4 computational thermochemistry protocol, and several post-W4 methods, have been applied to the P2 and P4 molecules. Contrary to previous studies, we find the experimental thermochemistry to be fundamentally sound. The reaction enthalpy for P4→2P2 has a very significant contribution from post-CCSD(T) correlation effects. We derive a gas-phase heat of formation for the phosphorus atom of [image omitted] kcal mol-1 and [image omitted] kcal mol-1, in the upper half of the CODATA uncertainty interval. [ABSTRACT FROM AUTHOR]

A computer program has been written to perform analytic derivative calculations of magnetic response properties based on uncontracted multireference configuration interaction (MRCI) wavefunctions. At this point, orbital relaxation is neglected in this treatment and the basis functions are assumed to be independent of the perturbation. It is therefore equivalent to an untruncated sum-over-states (SOS) approach to the g-tensor. Hence, the treatment is more rigorous than all previous MRCI g-tensor calculations that have used truncated SOS formulations. In the present calculations, MRCI calculations were performed on top of full valence complete-active space self-consistent field (CASSCF) reference wavefunctions without truncation of the reference or correlation spaces. Results are reported for two types of spin-orbit coupling operators, a simple effective nuclear charge approximation and the spin-orbit mean field (SOMF) approximation. The effects of orbital relaxation are discussed. The values obtained can be used in future calibration studies. [ABSTRACT FROM AUTHOR]

An implementation of the projected gradient method for locating the minimum energy crossing point between electronic states of different symmetry/multiplicity within the equation-of-motion coupled-cluster family of methods is reported. The method is applied to characterize the intersections between electronic states in [image omitted], NO2, and para-benzyne using the excitation energies, ionization potential, and spin-flip variants of the equation-of-motion coupled-cluster methods. The performance of the algorithm is discussed and recommendations for improving the convergence in problematic situations are given. [ABSTRACT FROM AUTHOR]

An IC-MRCI ansatz has been used in conjunction with augmented quadruple-ζ basis sets to construct an 89-point potential energy surface of the ground electronic state of [image omitted]. The equilibrium structure was predicted to be of C2v symmetry, possessing Be-H bond lengths of 1.609 Å and a H-Be-H bond angle of 29.4°. A rational Padé analytical function was fitted to the discrete IC-MRCI grid yielding a [image omitted] value of 5.20 cm-1. It was embedded in the t-coordinate Eckart-Watson Hamiltonian to calculate vibrational and rovibrational wave functions. For the vibrational ground state, the vibration-averaged bond lengths and angle were 1.647 Å and 29.2°, respectively. An embedded analytical IC-MRCI dipole moment function fitted to 73 discrete points was transformed to the space-fixed frame. Rovibrational radiative properties were calculated using this surface in conjunction with a novel integration scheme. [ABSTRACT FROM AUTHOR]

The reaction pathways to [image omitted] (tolyl, benzyl, and tropylium) from o-, m-, and p-chlorotoluene radical cations have been studied at the SCF level of theory using the DZ and DZP basis sets. The equilibrium structures and relative energies for the local minima and transition states have been fully optimized and the harmonic vibrational frequencies have been evaluated to differentiate the local minimum and the transition state. The lowest barrier rearrangement pathway is the formation of the benzyl cation for all three isomers, corroborating the recent experimental findings of Kim and Shin. This benzyl channel involves [1,3] α-hydrogen migration at the entrance and the activation energy is calculated to be 40.3, 39.7, and 42.2 kcal mol-1 for the o-, m-, and p-isomer, respectively. The other competing rearrangement pathway involves [1,2] α-hydrogen migration at the entrance, which leads to the tropylium ion, and the activation barrier is predicted to be 55.5, 54.2, and 56.62 kcal mol-1 for the o-, m-, and p-isomer, respectively. The highest barrier process involves direct C-Cl bond cleavage that yields the tolyl cation with a 0 K dissociation energy of 61.6, 64.6, and 68.92 kcal mol-1 for the o-, m-, and p-isomer, respectively. [ABSTRACT FROM AUTHOR]

Analytic energy gradients with respect to atomic coordinates for systems with translational invariance are formulated within the framework of Kohn-Sham Density Functional Theory. The energy gradients are implemented in the BAND program for periodic DFT calculations which directly employs a Bloch basis set made up of Slater-type (STOs) and numeric atomic orbitals (NAOs). The details of our implementation are described including the use of symmetry in the reciprocal and direct spaces, as well as the application of the frozen core approximation. [ABSTRACT FROM AUTHOR]

The coupled-cluster singles and doubles treatment with a perturbative treatment of triple excitations known as CCSD(T) has been used in conjunction with a hierarchy of atomic natural orbital basis sets to study stretching levels in water up to 2 eV above the zero-point level. Agreement with experiment obtained with perturbation theory augmented by a simple treatment of resonances suggested by Lehmann is quite remarkable in spite of the well-known and strong Darling-Dennison resonances in this system. With the largest basis sets, deviation from experiment is ca. 20 cm-1 at energies in the 15,000-16,000 cm-1 range. [ABSTRACT FROM AUTHOR]

In this work we examine three water models, including one developed by us, with distributed polarizable sites. These models are assessed by comparison with the results of MP2 calculations. The use of distributed polarizable sites is found to be especially important for describing the 3-body interaction energies of water clusters. It is also shown that all-atom repulsion terms are necessary to accurately describe cluster geometries. [ABSTRACT FROM AUTHOR]

The chelotropic addition reaction of singlet methylene to ethene yielding cyclopropane (reaction 1) was investigated with the help of the Unified Reaction Valley approach (URVA) using different levels of theory (B3LYP, MP2, MP4, CCSD(T), G3) and two basis sets (6-31G(d,p), 6-311++G(3df,3pd)). At all levels of theory, reaction (1) proceeds without barrier and transition state (TS). Nevertheless, reaction (1) possesses a distinct mechanism comprising four different reaction phases: (i) a van der Waals phase, in which the stereochemistry of the reaction is decided; (ii) an electrophilic attack phase, in which charge is transferred from ethene to methylene to establish a weak bonding interaction between the reaction partners typical of those encountered in TSs of CC bond forming reactions; (iii) a nucleophilic attack phase, in which charge transfer between methylene and ethene is reverted and a trimethylene biradical structure is formed; (iv) a ring closure phase, in which the trimethylene structure closes to the three-membered ring. The URVA analysis identifies a hidden TS and two hidden intermediates at the transitions from one phase to the next. If methylene is replaced by difluorocarbene (reaction 2) or germylene (reaction 3), the 4-phase mechanism is retained, however the hidden TS and one of the hidden intermediates are converted into a real TS and a real intermediate thus establishing 2-step mechanisms with strongly different energy profiles along the reaction path. [ABSTRACT FROM AUTHOR]

In this work, we present the analytical gradient of dual-basis second-order Møller-Plesset perturbation theory within the resolution-of-the-identity approximation (DB-RI-MP2). Interestingly, analytical DB-RI-MP2 gradient theory involves significant changes to both the theory and computation of the coupled-perturbed self-consistent field equations (CPSCF). From a theoretical point of view, the number of orbital responses required in DB-RI-MP2 analytical gradient theory has been reduced to the product of the number of occupied and virtual orbitals determined by the rank of the small atomic orbital (AO) basis. From a computational point of view, the DB-CPSCF equations can be solved within this smaller space at a fraction of the computational cost. Additional computational savings can be obtained during the digestion of the four-centered AO integral derivatives and the efficient underlying DB-SCF procedure, which lead to a significant overall reduction in the computational cost necessary for treating molecular systems containing less than 100 atoms. Based on stringent chemical tests and a detailed computational timings analysis, it was found that DB-RI-MP2 reproduces molecular equilibrium structures with an accuracy that approximates RI-MP2 using the target AO basis but at a dramatically reduced cost, thereby enabling more routine use of large AO basis sets during geometry optimizations at the MP2 level of theory. [ABSTRACT FROM AUTHOR]

Empirical valence bond (EVB) surfaces have been constructed for 2-pyridone-2-hydroxypyridine proton transfer and for the Claisen rearrangement of allyl vinyl ether at the MP2/6-311 + G(d,p) level of theory. A distributed Gaussian approach is used to approximate the interaction matrix elements. Parameters for the distributed Gaussians are determined by fitting to energy, gradient and Hessian data obtained from ab initio electronic structure calculations at one to nine points along the reaction path. An efficient DIIS (direct inversion of iterative subspace) method is used to solve the fitting equations. Criteria for choosing internal coordinates for the representation of the potential energy surfaces and for the interaction matrix element are discussed. Practical techniques for determining the placement and exponents of the Gaussians are described. With one set of s-, p- and d-type Gaussians at the transition state, the error in the energy along the reaction path is less than 10 kJ mol-1 for pyridone tautomerization. Five sets of Gaussians reduces the error to less than 5 kJ mol-1 and seven Gaussians drops the error below 1 kJ mol-1. The Claisen rearrangement is more challenging and requires seven Gaussians to achieve an error of less than 4 kJ mol-1 for energies along the reaction path. [ABSTRACT FROM AUTHOR]

Natural localized molecular orbitals (NLMOs) have been used in local correlation calculations. For a test set of 30 molecules it is shown that the results obtained with NLMOs and Pipek-Mezey localization are very similar. However, NLMOs are much less sensitive to the basis set, in particular when diffuse functions are used. Based on natural population analysis (NPA) a new method to determine the domains in local correlation methods is proposed. It is demonstrated that this yields domains that are very insensitive to the choice of the basis set. For all 30 molecules the same domains are obtained with six different basis sets ranging from cc-pVDZ to aug-cc-pVQZ. This allows one to define the local correlation methods more uniquely than with previous methods. [ABSTRACT FROM AUTHOR]

High-level ab initio calculations with relatively large basis sets are reported for bromofluoromethane, CH2BrF. Correlated harmonic and anharmonic force constants are obtained from several levels of theory including second-order Møller-Plesset perturbation and coupled cluster CCSD(T). Density functional theory and to be more precise the B3LYP functional, is also employed. Results are used to determine a number of anharmonic molecular properties, including vibration-rotation interaction constants, vibrational anharmonicity constants, fundamental vibrational frequencies, sextic centrifugal distortion constants and rotational constants which include zero-point corrections. Good agreement with available experimental data is found combining coupled cluster CCSD(T) optimized geometry and harmonic force field with cubic and quartic force constants obtained at MP2 level of theory, a cost-effective computation in terms of time. Moreover, transformations of the force fields in mass-independent internal coordinate force constants suitable for other isotopologues of CH2BrF are also investigated. [ABSTRACT FROM AUTHOR]