Testing the quasicentroid molecular dynamics method on gas-phase ammonia

TL;DR

This study evaluates the quasicentroid molecular dynamics method's effectiveness in simulating gas-phase ammonia's infrared spectrum, demonstrating its potential to accurately incorporate nuclear quantum effects beyond water.

Contribution

It extends the application of QCMD to ammonia, showing it performs comparably to water and highlighting its limitations with highly anharmonic modes.

Findings

QCMD reduces blue shifts in ammonia's spectrum.

QCMD performs similarly for ammonia and water.

Limited improvement for highly anharmonic modes.

Abstract

Quasicentroid molecular dynamics (QCMD) is a path-integral method for approximating nuclear quantum effects in dynamics simulations, which has given promising results for gas- and condensed-phase water. Here, by simulating the infrared spectrum of gas-phase ammonia, we test the feasibility of extending QCMD beyond water. Overall, QCMD works as well for ammonia as for water, reducing or eliminating blue shifts from the classical spectrum without introducing the artificial red-shifts or broadening associated with other imaginary-time path-integral methods. However, QCMD gives only a modest improvement over the classical spectrum for the position of the symmetric bend mode, which is highly anharmonic (since it correlates with the inversion pathway). We expect QCMD to have similar problems with large-amplitude degrees of freedom in other molecules, but otherwise to work as well as for water.