An Embedded Fragment Method for Molecules in Strong Magnetic Fields

Abstract

An extension of the embedded fragment method for calculations on molecular clusters is presented, which includes strong external magnetic fields. The approach is flexible, allowing for calculations at the Hartree−Fock, current-density-functional theory, Møller−Plesset perturbation theory, and coupled-cluster levels using London atomic orbitals. For systems consisting of discrete molecular subunits, calculations using London atomic orbitals can be performed in a computationally tractable manner for systems beyond the reach of conventional calculations, even those accelerated by resolution-of-the-identity or Cholesky decomposition methods. To assess the applicability of the approach, applications to water clusters are presented, showing how strong magnetic fields enhance binding within the clusters. However, our calculations suggest that, contrary to previous suggestions in the literature, this enhanced binding may not be directly attributable to strengthening of hydrogen bonding. Instead, these results suggest that this arises for larger field strengths as a response of the system to the presence of the external field, which induces a charge density build up between the monomer units. The approach is embarrassingly parallel and its computational tractability is demonstrated for clusters of up to 103 water molecules in triple-ζ basis sets, which would correspond to conventional calculations with more than 12 000 basis functions.

Molecular charge distributions in strong magnetic fields: a conceptual and current DFT study

“…Future work considering the change in charge distributions of water dimers and possibly larger systems, informed by the work of Ref. [48], should help provide additional insight to the unresolved question of the overall effect of magnetic fields on hydrogen bonding.…”

“…Theoretical studies on the dynamics of liquid water [53] or on the properties of the water dimer in magnetic fields [54] have investigated the link between these phenomena and hydrogen bonding but present a somewhat mixed picture. A recent study by Speake et al on the behaviour of large water clusters in magnetic fields confirms the increase in interaction energy with field strength, however analysis of the accompanying change in charge density does not suggest increased strength of hydrogen bonding as the predominant reason for this [48]. It is apparent that further study will be necessary to better understand the changes in the properties of water that are observed when a magnetic field is applied [55].…”

“…Subsequent work, with the adaptation of other electronic structure methods to systems in strong magnetic fields in London and in other electronic structure packages, including Turbomole, [41] Qcumbre, [42] Bagel [43] and the Quest code used in the present work, [44] has significantly broadened the range of chemistry that can be investigated under these extreme conditions. Recent studies have considered the effects of strong magnetic fields on the absorption spectra, [45] equilibrium geometries [46] and excitation energies [47] of atoms and molecules and the interaction energy of large molecular clusters [48] under these conditions. The work by Deb and colleagues has concentrated on electron dynamics in small atoms and molecules under the influence of strong oscillating magnetic fields using a fundamentally different quantum field density functional theory approach, details of which may be found in Ref.…”

Semiempirical Methods for Molecular Systems in Strong Magnetic Fields

“…In recent years, a large number of approaches have been developed to allow for the study of atomic and molecular systems in strong magnetic fields. − Several electronic structure packages have now been developed with the capabilities to allow a nonperturbative treatment of magnetic fields − for a range of methods including Hartree–Fock (HF), , configuration interaction, Møller–Plesset (MP), coupled-cluster (CC), equation of motion coupled-cluster, , Green’s function-based GW, , and current-density functional theories (CDFTs). ,,,,, …”

“…Given that the effects of magnetic fields are expected to be observable at lower fields for larger systems, it is desirable to develop lower-cost approaches to increase the system size that is amenable to simulation. Recently, embedded fragment-based approaches were developed based on HF, CDFT, MP, and CC methods with LAO basis sets to address large, noncovalently bound, molecular clusters …”