posted on 2021-11-01, 18:41authored byJoseph
P. Heindel, Sotiris S. Xantheas
We
present a protocol for classical and nuclear quantum dynamics,
in which the energies and forces are generated by the many-body expansion
(MBE), and apply it to water clusters using the TTM2.1-F and MB-Pol
interaction potentials at various temperatures. We carry out MBE-molecular
dynamics (MD) classical and nuclear quantum dynamical simulations,
in which the energies and forces of the full system are approximated
by the two-, three-, and four-body terms of the MBE, and compare the
average potential and the vibrational density of states with the full
simulation, i.e., the one for which no MBE is used. Our results indicate that the thermally averaged
potential energy from the MBE up to the four-body term converges with
near-identical behavior to the one from the full simulation. The three-body
makes a substantial contribution (∼20%) to the energy, whereas
the four-body is necessary for obtaining quantitatively accurate energetics
and forces, albeit making a small contribution to each (∼2%).
We further show that the harmonic frequencies are reproduced to within
a few wavenumbers (cm–1) at the four-body level
and that the slowest modes to converge with the MBE rank are those
involving the strongest hydrogen bonds. Anharmonicity exacerbates
this effect, so that a four-body description of the energies and forces
is needed to achieve accurate anharmonic vibrational frequencies in
the hydrogen-bonded OH-stretching region. We also discuss the asymptotic
scaling of the MBE-MD protocol with respect to the cost of the underlying
potential energy evaluation, suggesting that electronic structure
methods that scale at least as N4, N being the size of the system, are needed to result in
savings over the traditional full MD simulation. We anticipate that
the MBE-MD protocol can evolve into a powerful and practical method,
which will allow for highly accurate ab initio MD simulations on a
much broader range of molecular systems than can be currently handled.