A nuclear configuration interaction approach to study nuclear spin effects: an application to ortho- and para-3 He2 @C60.

We introduce a non-orthogonal configuration interaction approach to investigate nuclear quantum effects on energies and densities of confined fermionic nuclei. The Hamiltonian employed draws parallels between confined systems and many-electron atoms, where effective non-Coulombic potentials represent the interactions of the trapped particles. One advantage of this method is its generality, as it offers the potential to study the nuclear quantum effects of various confined species affected by effective isotropic or anisotropic potentials. As a first application, we analyze the quantum states of two 3 He atoms encapsulated in C60 . At the Hartree-Fock level, we observe the breaking of spin and spatial symmetries. To ensure wavefunctions with the correct symmetries, we mix the broken-symmetry Hartree-Fock states within the non-orthogonal configuration interaction expansion. Our proposed approach predicts singly and triply degenerate ground states for the singlet (para-3 He2 @C60 ) and triplet (ortho-3 He2 @C60 ) nuclear spin configurations, respectively. The ortho-3 He2 @C60 ground state is 5.69 cm-1 higher in energy than the para-3 He2 @C60 ground state. The nuclear densities obtained for these states exhibit the icosahedral symmetry of the C60 embedding potential. Importantly, our calculated energies for the lowest 85 states are in close agreement with perturbation theory results based on a harmonic oscillator plus rigid rotor model of 3 He2 @C60 .

The any particle molecular orbital grid-based Hartree-Fock (APMO-GBHF) approach.

The any particle molecular orbital grid-based Hartree-Fock approach (APMO-GBHF) is proposed as an initial step to perform multi-component post-Hartree-Fock, explicitly correlated, and density functional theory methods without basis set errors. The method has been applied to a number of electronic and multi-species molecular systems. Results of these calculations show that the APMO-GBHF total energies are comparable with those obtained at the APMO-HF complete basis set limit. In addition, results reveal a considerable improvement in the description of the nuclear cusps of electronic and non-electronic densities.

Negative muon chemistry: the quantum muon effect and the finite nuclear mass effect.

The any-particle molecular orbital method at the full configuration interaction level has been employed to study atoms in which one electron has been replaced by a negative muon. In this approach electrons and muons are described as quantum waves. A scheme has been proposed to discriminate nuclear mass and quantum muon effects on chemical properties of muonic and regular atoms. This study reveals that the differences in the ionization potentials of isoelectronic muonic atoms and regular atoms are of the order of millielectronvolts. For the valence ionizations of muonic helium and muonic lithium the nuclear mass effects are more important. On the other hand, for 1s ionizations of muonic atoms heavier than beryllium, the quantum muon effects are more important. In addition, this study presents an assessment of the nuclear mass and quantum muon effects on the barrier of Heµ + H2 reaction.

Including nuclear quantum effects into highly correlated electronic structure calculations of weakly bound systems.

An interface between the APMO code and the electronic structure package MOLPRO is presented. The any particle molecular orbital APMO code [González et al., Int. J. Quantum Chem. 108, 1742 (2008)] implements the model where electrons and light nuclei are treated simultaneously at Hartree-Fock or second-order Möller-Plesset levels of theory. The APMO-MOLPRO interface allows to include high-level electronic correlation as implemented in the MOLPRO package and to describe nuclear quantum effects at Hartree-Fock level of theory with the APMO code. Different model systems illustrate the implementation: (4)He2 dimer as a protype of a weakly bound van der Waals system; isotopomers of [He-H-He](+) molecule as an example of a hydrogen bonded system; and molecular hydrogen to compare with very accurate non-Born-Oppenheimer calculations. The possible improvements and future developments are outlined.

A generalized any particle propagator theory: assessment of nuclear quantum effects on electron propagator calculations.

In this work we propose an extended propagator theory for electrons and other types of quantum particles. This new approach has been implemented in the LOWDIN package and applied to sample calculations of atomic and small molecular systems to determine its accuracy and performance. As a first application of the method we have studied the nuclear quantum effects on electron ionization energies. We have observed that ionization energies of atoms are similar to those obtained with the electron propagator approach. However, for molecular systems containing hydrogen atoms there are improvements in the quality of the results with the inclusion of nuclear quantum effects. An energy term analysis has allowed us to conclude that nuclear quantum effects are important for zero order energies whereas propagator results correct the electron and electron-nuclear correlation terms. Results presented for a series of n-alkanes have revealed the potential of this method for the accurate calculation of ionization energies of a wide variety of molecular systems containing hydrogen nuclei. The proposed methodology will also be applicable to exotic molecular systems containing positrons or muons.

Optimización de paquete computacional para el cálculo de estructura núcleo-electrónica: apmo / Opimization of ssoftware package for studies of quantum nuclear effect: apmo / Otimiação do proograma computacional ara o cálculo da estrutura elettrônica e não eletrônica: apmo

En este trabajo se presenta una versión rediseñada y optimizada del programa APMO (Any Particle Molecular Orbital). APMO es una implementación computacional del método del orbital molecular nuclear y electrónico (OMNE) en niveles de teoría Hartree-Fock (HF) y de perturbaciones de muchos cuerpos de segundo orden (MP2). En el método OMNE, tanto los núcleos atómicos como electrones se representan como funciones de onda. Este método permite un estudio directo de fenómenos en los que se presentan efectos cuánticos nucleares: efectos isotópicos, deslocalización nuclear, transferencia de protones, entre otros. La optimización realizada logró una marcada disminución en los tiempos de un cálculo global y permitió el uso de funciones de base electrónicas y nucleares con altos momentos angulares. Como ejemplo de las nuevas posibilidades del programa se presenta un estudio del efecto isotópico en complejos monohidratados y dihidratados de cobre (I) y cinc (II). En estos sistemas se encontró que la sustitución de protio por deuterio debilita el enlace oxígeno-metal.

This paper describes the optimization of the overall calculation scheme and the implementations of an efficient system for calculate molecular integrals in the APMO software package (Any Particle Molecular Orbital). APMO is an implementation of the nuclear and electronic molecular orbital (NEMO) method at Hatree-Fock (HF) and MP2 levels of theory. In this method, both nuclei and electrons are represented as wave functions, which allow the study of phenomena where nuclear quantum effects are important, such as isotope effects, hydrogen bonding, proton transfer, and others. This optimization reached a marked decrease in global and molecular integrals calculation times and enabled the use ofbasis functions with angular momenta higher than d and allowed the calculation of systems with more than eight atoms. This paper also presents the application of the NEMO method to the study of the isotope effect on mono and dihydrated complexes of copper (I) and zinc (II). For these systems, we found that the substitution of a proton with a deuteron nucleus weakens the metal-oxygen bond.

Este artigo descreve a otimização do sistema no cálculo global e a implementação de um sistema eficiente para calcular integrais moleculares no programa computacional APMO (Any Particle Molecular Orbital). APMO é uma implementação do método dos orbitais moleculares nucleares e eletrónicos (OMNE) no nível da teoria Hartree-Fock (HF) e o MP2. Neste método, tanto núcleos como elétrons apresentam-se como funções de onda permitirem o estudo dos fenômenos nucleares onde os efeitos quânticos são importantes, tais como efeitos de isótopos, pontes de hidrogênio, transferência de prótons, e outros. Com a otimização realizada, diminuiu o tempo do cálculo global e de integrais molecular; além disso, permitiu a utilização de funções de base com o momento angular superior a d eo cálculo de sistemas com mais de oito átomos. Este trabalho apresenta também a aplicação do método e NEMO para o estudo do efeito isotópico em mono-complexos e di-complexos de cobre (I) e zinco (II). Para estes sistemas, descobrimos que a substituição de um próton com um núcleo deuteron enfraquece o vínculo de oxigênio-metal.