Bonding in the helium dimer in strong magnetic fields: the role of spin and angular momentum

David

Teale

2021

An efficient implementation of geometrical derivatives at the Hartree–Fock (HF) and current-density functional theory (CDFT) levels is presented for the study of molecular structure in strong magnetic fields. The required integral derivatives are constructed using a hybrid McMurchie–Davidson and Rys quadrature approach, which combines the amenability of the former to the evaluation of derivative integrals with the efficiency of the latter for basis sets with high angular momentum. In addition to its application to evaluating derivatives of four-center integrals, this approach is also applied to gradients using the resolution-of-the-identity approximation, enabling efficient optimization of molecular structure for many-electron systems under a strong magnetic field. The CDFT contributions have been implemented for a wide range of density functionals up to and including the meta-GGA level with current-density dependent contributions and (range-separated) hybrids for the first time. Illustrative applications are presented to the OH and benzene molecules, revealing the rich and complex chemistry induced by the presence of an external magnetic field. Challenges for geometry optimization in strong fields are highlighted, along with the requirement for careful analysis of the resulting electronic structure at each stationary point. The importance of correlation effects is examined by comparison of results at the HF and CDFT levels. The present implementation of molecular gradients at the CDFT level provides a cost-effective approach to the study of molecular structure under strong magnetic fields, opening up many new possibilities for the study of chemistry in this regime.

Section: Resultsmentioning

confidence: 99%

Optimizing Molecular Geometries in Strong Magnetic Fields

David

Teale

2021

“…For magnetic fields aligned perpendicular to the internuclear axis, the symmetry is reduced to C 2 h , having a C 2 axis perpendicular to bond axis and along the field direction, a σ h in the plane perpendicular to the axis containing the two nuclei and a center of inversion i . For all other orientations of the magnetic field, the symmetry is reduced to C i , with only the center of inversion preserved. , The molecular orbitals then belong to the irreducible representations of these point groups, rather than those of the D ∞ h point group. In Table , the molecular orbital labels are given for N 2 in each of these point groups in order of the zero-field ground state configuration predicted at the cTPSSrsh/6-311++G** level, as discussed in Section , for the first 26 orbitals.…”

Section: Resultsmentioning

confidence: 99%

Modeling Ultrafast Electron Dynamics in Strong Magnetic Fields Using Real-Time Time-Dependent Electronic Structure Methods

Wibowo

Teale

2021

An implementation of real-time time-dependent Hartree–Fock (RT-TDHF) and current density functional theory (RT-TDCDFT) for molecules in strong uniform magnetic fields is presented. In contrast to earlier implementations, the present work enables the use of the RT-TDCDFT formalism, which explicitly includes field-dependent terms in the exchange–correlation functional. A range of current-dependent exchange–correlation functionals based on the TPSS functional are considered, including a range-separated variant, which is particularly suitable for application to excited state calculations. The performance of a wide range of propagator algorithms for real-time methods is investigated in this context. A recently proposed molecular orbital pair decomposition analysis allows for assignment of electronic transitions, providing detailed information about which molecular orbitals are involved in each excitation. The application of these methods is demonstrated for the electronic absorption spectra of N 2 and H 2 O both in the absence and in the presence of a magnetic field. The dependence of electronic spectra on the magnetic field strength and its orientation relative to the molecule is studied. The complex evolution of the absorption spectra with magnetic field is rationalized using the molecular orbital pair decomposition analysis, which provides crucial insight in strong fields where the spectra are radically different from their zero-field counterparts.

“…This results in the presence of a minimum at B ⊥ ≈ 0.1 B 0 . The effect of a perpendicular magnetic field on these states has been already discussed by Hampe and Stopkowicz, and the reader should refer to refs , , and for a deeper analysis. In our comparison, the MOM correctly represents the energy changes with respect to the magnetic field strength, and it may be seen that the crossings of the 1 A g state with the 3 B u and 3 A u states are qualitatively well represented.…”

Section: Results and Discussionmentioning

confidence: 84%

Self-Consistent Field Methods for Excited States in Strong Magnetic Fields: a Comparison between Energy- and Variance-Based Approaches

David

Fouda

et al. 2021

Self-consistent field methods for excited states offer an attractive low-cost route to study not only excitation energies but also properties of excited states. Here, we present the generalization of two selfconsistent field methods, the maximum overlap method (MOM) and the σ-SCF method, to calculate excited states in strong magnetic fields and investigate their stability and accuracy in this context. These methods use different strategies to overcome the well-known variational collapse of energybased optimizations to the lowest solution of a given symmetry. The MOM tackles this problem in the definition of the orbital occupations to constrain the self-consistent field procedure to converge on excited states, while the σ-SCF method is based on the minimization of the variance instead of the energy. To overcome the high computational cost of the variance minimization, we present a new implementation of the σ-SCF method with the resolution of identity approximation, allowing the use of large basis sets, which is an important requirement for calculations in strong magnetic fields. The accuracy of these methods is assessed by comparison with the benchmark literature data for He, H 2 , and CH + . The results reveal severe limitations of the variance-based scheme, which become more acute in large basis sets. In particular, many states are not accessible using variance optimization. Detailed analysis shows that this is a general feature of variance optimization approaches due to the masking of local minima in the optimization. In contrast, the MOM shows promising performance for computing excited states under these conditions, yielding results consistent with available benchmark data for a diverse range of electronic states.