Edward T. Seidl

Psi4: an open-source ab initio electronic structure program

The Psi4 program is a new approach to modern quantum chemistry, encompassing Hartree–Fock and density‐functional theory to configuration interaction and coupled cluster. The program is written entirely in C++ and relies on a new infrastructure that has been designed to permit high‐efficiency computations of both standard and emerging electronic structure methods on conventional and high‐performance parallel computer architectures. Psi4 offers flexible user input built on the Python scripting language that enables both new and experienced users to make full use of the programs capabilities, and even to implement new functionality with moderate effort. To maximize its impact and usefulness, Psi4 is available through an open‐source license to the entire scientific community.

PSI3: An open‐source Ab Initio electronic structure package

PSI3 is a program system and development platform for ab initio molecular electronic structure computations. The package includes mature programming interfaces for parsing user input, accessing commonly used data such as basis‐set information or molecular orbital coefficients, and retrieving and storing binary data (with no software limitations on file sizes or file‐system‐sizes), especially multi‐index quantities such as electron repulsion integrals. This platform is useful for the rapid implementation of both standard quantum chemical methods, as well as the development of new models. Features that have already been implemented include Hartree‐Fock, multiconfigurational self‐consistent‐field, second‐order Møller‐Plesset perturbation theory, coupled cluster, and configuration interaction wave functions. Distinctive capabilities include the ability to employ Gaussian basis functions with arbitrary angular momentum levels; linear R12 second‐order perturbation theory; coupled cluster frequency‐dependent response properties, including dipole polarizabilities and optical rotation; and diagonal Born‐Oppenheimer corrections with correlated wave functions. This article describes the programming infrastructure and main features of the package. PSI3 is available free of charge through the open‐source, GNU General Public License.

Accurate structures and binding energies for small water clusters: The water trimer

The global minimum on the water trimer potential energy surface has been investigated by means of second-order Mo/ller-Plesset (MP2) perturbation theory employing the series of correlation-consistent basis sets aug-cc-pVXZ (X = D, T, Q, 5, 6), the largest of which contains 1329 basis functions. Definitive predictions are made for the binding energy and equilibrium structure, and improved values are presented for the harmonic vibrational frequencies. A value of 15.82±0.05 kcal mol−1 is advanced for the infinite basis set frozen core MP2 binding energy, obtained by extrapolation of MP2 correlation energies computed at the aug-cc-pVQZ MP2 geometry. Inclusion of core correlation, using the aug-cc-pCV5Z basis set, has been found to increase the binding energy by 0.08 kcal mol−1, and after consideration of core correlation and higher-order correlation effects, the classical binding energy for the water trimer is estimated to be 15.9±0.2 kcal mol−1. A zero-point vibrational correction of −5.43 kcal mol−1 has bee...

Is there a transition state for the unimolecular dissociation of cyclotetraoxygen (O4)

The cyclo‐O4 molecule is isoelectronic with cyclobutane and has been mentioned as a potential new high energy density material (HEDM). The important unresolved question has been whether or not cyclo‐O4 is a genuine minimum on the O4 potential energy hypersurface. Here the transition state for cyclo‐O4 dissociation to two O2 molecules has been located at a number of levels of theory using a double zeta plus polarization (DZP) basis set. The theoretical methods with which the transition state geometry was optimized include two‐configuration self‐consistent‐field (TCSCF), configuration interaction including all single and double excitations (CISD), coupled cluster including all single and double excitations (CCSD), and the latter with a noniterative correction for connected triple excitations, CCSD(T). The equilibrium geometry of O4 has D2d symmetry, while a stationary point of D2 symmetry is of Hessian index two (i.e., two imaginary vibrational frequencies) at the highest level of theory, DZP CCSD(T). The t...

Equilibrium geometry of the HCCN triplet ground state : carbene or allene ? An open-shell coupled cluster study including connected triple excitations

The molecular structure of the HCCN molecule has been investigated using open‐shell coupled cluster methods. The full single and double excitation procedure (CCSD) is used initially. Connected triple excitations were then included perturbatively via the CCSD(T) method. A standard double zeta plus polarization (DZP) basis set, designated H(4s1p/2s1p), C,N(9s5p1d/4s2p1d), was used initially. The predicted carbene H–C–C equilibrium bond angles are 133.8° (CISD), 138.4° (CCSD), and 139.8° with the CCSD(T) method. The corresponding bent‐linear energy differences are 5.30, 2.70, and 2.15 kcal/mol, respectively. A much larger basis set was chosen to consider the importance of d functions on hydrogen and f functions on the carbon and nitrogen atoms. Using this H(5s2p1d/3s2p1d), C,N(10s6p2d1f/5s3p2d1f) basis, the H–C–C equilibrium bond angle is 136.3° with the CISD method and the bent‐linear energy difference 3.8 kcal/mol. Best estimates of the HCC equilibrium bond angle and singlet–triplet separation are 142° and 0.8 kcal/mol, respectively. The relationship of these theoretical results to several experiments is discussed.

Acetylene: Synergy between theory and experiment

Six anomalous vibronic feature states [∼2 cm−1 full‐width at half‐maximum (FWHM), each consisting of ∼20 partially resolved eigenstates] have been observed in stimulated emission pumping (SEP) spectra of C2D2. Of the two plausible assignments for these features, the one most consistent with spectroscopic observations would imply that the lowest energy cis‐bent triplet state of acetylene has T0≤25 820 cm−1, which is inconsistent with previous ab initio predictions. New higher level ab initio quantum mechanical methods have been used to predict the energy difference between X 1Σg+ ground state and the cis‐bent a 3B2 lowest triplet state of acetylene. In conjunction with a triple zeta plus double polarization plus f function (TZ2Pf) basis set, the coupled cluster including single, double, and linearized triple excitations CCSD(T) method yields T0=ΔE(a 3B2–X 1Σg+)=30 500 cm−1. The true value of T0 for the a 3B2 state is estimated to be ∼500 cm−1 higher. At the same level of theory the zero‐point levels o...

The titane molecule (TiH4): Equilibrium geometry, infrared and Raman spectra of the first spectroscopically characterized transition metal tetrahydride

The titanium tetrahydride molecule has been investigated using several levels of ab initio molecular electronic structure theory in conjunction with a basis set of size Ti(14s11p6d/10s8p3d), H(5s1p/3s1p). The self‐consistent field, single and double excitation configuration interaction, and single and double excitation coupled cluster methods were used to predict the molecular structure, harmonic vibrational frequencies, infrared and Raman intensities. Results for all five isotopic species TiH4, TiH3D, TiH2D2, TiHD3, and TiD4 are presented. A careful comparison with the recent matrix isolation infrared results of Xiao, Hauge, and Margrave is made where experimental data exist. All four of the tentatively assigned experimental vibrational features are given strong support from theory.