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DL_POLY Quantum 2.1 is introduced here as a highly modular, sustainable, and scalable general-purpose molecular dynamics (MD) simulation software for large-scale long-time MD simulations of condensed phase and interfacial systems with the essential nuclear quantum effects (NQEs) included. The new release improves upon version 2.0 through the introduction of several emerging real-time path integral (PI) methods, including fast centroid molecular dynamics (f-CMD) and fast quasi-CMD (f-QCMD) methods, as well as our recently introduced hybrid CMD (h-CMD) method for the accurate and efficient simulation of vibrational infrared spectra. Several test cases, including liquid bulk water at 300 K and ice Ih at 150 K, are used to showcase the performance of different implemented PI methods in simulating the infrared spectra at both ambient conditions and low temperatures where NQEs become more apparent. Additionally, using different salt-in-water (i.e., dilute) and water-in-salt (i.e., concentrated) lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) aqueous electrolyte solutions, we demonstrate the applicability of our recently introduced h-CMD method implemented in DL_POLY Quantum 2.1 for the large scale simulation of infrared (IR) spectra of complex heterogeneous systems. We show that h-CMD can overcome the curvature problem of CMD and the artificial broadening of T-RPMD for the accurate simulation of the vibrational spectra of complex, heterogeneous systems with NQEs included. 

Developing efficient path integral (PI) methods for atomistic simulations of vibrational spectra in heterogeneous condensed phases and interfaces has long been a challenging task. Here, we present the h-CMD method, short for hybrid centroid molecular dynamics, which combines the recently introduced fast quasi-CMD (f-QCMD) method with fast CMD (f-CMD). In this scheme, molecules that are believed to suffer more seriously from the curvature problem of CMD, e.g., water, are treated with f-QCMD, while the rest, e.g., solid surfaces, are treated with f-CMD. To test the accuracy of the newly introduced scheme, the infrared spectra of the interfacial D2O confined in the archetypal ZIF-90 framework are simulated using h-CMD compared to a variety of other PI methods, including thermostatted ring-polymer molecular dynamics (T-RPMD) and partially adiabatic CMD as well as f-CMD and experiment as reference. Comparisons are also made with classical MD, where nuclear quantum effects are neglected entirely. Our detailed comparisons at different temperatures of 250–600 K show that h-CMD produces O–D stretches that are in close agreement with the experiment, correcting the known curvature problem and redshifting of the stretch peaks of CMD. h-CMD also corrects the known issues associated with too artificially dampened and broadened spectra of T-RPMD, which leads to missing the characteristic doublet feature of the interfacial confined water, rendering it unsuitable for these systems. The new h-CMD method broadens the applicability of f-QCMD to heterogeneous condensed phases and interfaces, where defining curvilinear coordinates for the entire system is not feasible. 

DL_POLY Quantum 2.0, a vastly expanded software based on DL_POLY Classic 1.10, is a highly parallelized computational suite written in FORTRAN77 with a modular structure for incorporating nuclear quantum effects into large-scale/long-time molecular dynamics simulations. This is achieved by presenting users with a wide selection of state-of-the-art dynamics methods that utilize the isomorphism between a classical ring polymer and Feynman’s path integral formalism of quantum mechanics. The flexible and user-friendly input/output handling system allows the control of methodology, integration schemes, and thermostatting. DL_POLY Quantum is equipped with a module specifically assigned for calculating correlation functions and printing out the values for sought-after quantities, such as dipole moments and center-of-mass velocities, with packaged tools for calculating infrared absorption spectra and diffusion coefficients.