Results for liquid water

Chapter II: Dimension-free path-integral molecular dynamics

2.8 Results for liquid water

Figure 2.4: Primitive and virial kinetic energy expectation valuesas a function of the bead number per hydrogen atom in liquid water at 298 K and 0.998 g/cm³ at timestep Δ𝑡 = 0.5 fs (a, c) andΔ𝑡 = 0.8 fs (b, d). The reference kinetic energy, obtained from a converged staging PIMD simulation at timestepΔ𝑡 = 0.1 fs and bead number 𝑛 =256, is indicated with a dashed line. The standard error of all visible data points in each plot is smaller than the symbol size.

obtained using the OBABO integrator in the small-timestep limit.

from the thermostat; for the remaining𝑛−1 internal modes, simulations performed with the OBABO and BAOAB schemes use the standard^27,29 damping schedule of 𝚪 = 𝛀, and simulations performed using the Cayley modification use friction 𝛾_𝑗 = min{𝜔_{𝑗 ,𝑛},0.9𝛾^max

𝑗 (𝜔²

OH),0.9𝛾^max

𝑗 (0)}, where 𝛾^max

𝑗 (Λ/𝑚) saturates the in- equality in Eq. (2.12) for the given values of 𝑗 and Λ/𝑚 at the given time step, and𝜔_OH is the OH-stretch frequency from the harmonic bending force field term in the q-TIP4P/F force field. Multi-nanosecond staging PIMD^25,31simulations at a timestep of 0.1 fs were performed to obtain a bead-converged reference value for the H-atom kinetic energy, plotted as a dashed line in Figs. 2.4 and 2.5.

The primitive kinetic energy expectation values in panels (a) and (b) of Fig. 2.4 show similar trends to those seen in Figs. 2.1 and 2.2 for the harmonic and weakly anhar- monic oscillators. For a 0.5-fs timestep (Fig. 2.4a), at which all integrators exhibit strong stability for ring polymers with up to 64 beads at the system temperature,⁶⁰ the OBABO, BAOAB, and OBCBO primitive kinetic energy estimates diverge from the converged result as the number of beads increases, in agreement with the proven result that the error in the ring-polymer configurational distribution generated with these schemes grows unboundedly with increasing bead number. At the larger, 0.8- fs timestep, (Fig. 2.4b), OBABO and BAOAB formally lose strong stability and their respective primitive kinetic energy estimates dramatically diverge for bead numbers greater than 32; the strongly stable OBCBO scheme also yields a divergent result for the same reason as in Fig. 2.4(a). As seen on the HO and aHO model systems, the primitive kinetic energy expectation value from the BCOCB integrator mono- tonically converges to the reference value with increasing bead number, avoiding any perceptible timestep error.

Fig. 2.4(c-d) shows the corresponding virial kinetic energy expectation values. For the smaller timestep of 0.5 fs, which is a common choice for path-integral simulations of water, all of the integrators perform similarly. However, upon increasing the timestep to 0.8 fs, significant differences in the performance of the integrators emerges, with only BCOCB avoiding perceptible timestep error.

To further compare the accuracy and stability of the OBABO, BAOAB, OBCBO, and BCOCB integrators, Fig. 2.5 considers the average kinetic energy per hydrogen atom obtained using 64 beads over a wide range of timesteps. These results show that BCOCB remains remarkably accurate for timesteps as large as 1.4 fs for liquid water, which corresponds to the limit of stability for Verlet integration of the centroid mode. In comparison, OBCBO diverges monotonically as the timestep increases, reaching unphysical values for the primitive expectation value and yielding sizable error (20%) for the virial expectation value. The erratic performance of both OBABO and BAOAB is due to the emergence of numerical resonance instabilities at timesteps greater than 0.6 fs at the employed bead number; indeed, the largest safe timestep at which OBABO and BAOAB remain strongly stable for 𝑛 = 64, Δ𝑡_★ ≈0.63 fs, precedes the range of timesteps in Fig. 2.5 for which these integrators

Figure 2.5: Primitive and virial kinetic energy expectation valuesas a function of the timestep per hydrogen atom in liquid water at 298 K and 0.998 g/cm³, as described by a 64-bead ring polymer. The reference kinetic energy, obtained from a converged staging PIMD simulation at timestep Δ𝑡 = 0.1 fs and bead number 𝑛 = 256, is indicated with a dashed line. The standard error of all visible data points in each plot is smaller than the symbol size.

vary erratically.

Extending beyond statistics, we now consider the dynamical properties of liquid water. Given the superiority of the BCOCB scheme for the calculated statistical properties in Figs. 2.4 and 2.5, we present results that focus on this scheme in com- parison to the most widely used OBABO scheme. In particular, we consider the liquid water infrared absorption spectrum,¹¹which is proportional to𝜔²𝐼˜(𝜔)where the dipole spectrum ˜𝐼(𝜔) =∫

Rd𝑡 𝑒^{−𝑖𝜔𝑡}𝐶˜_𝜇·𝜇(𝑡)is the Fourier transform of the Kubo- transformed dipole autocorrelation function ˜𝐶_𝜇·𝜇(𝑡). The latter is approximated in the RPMD model by⁹ 𝐶˜_𝜇·𝜇(𝑡) = _𝑁¹ Í𝑁

𝑖=1⟨𝜇¯_𝑖(𝑡) ·𝜇¯_𝑖(0)⟩, where 𝑁 is the number of molecules in the liquid, ¯𝜇_𝑖(𝑡) is the bead-averaged dipole moment of molecule 𝑖 at time 𝑡, and the angle brackets denote averaging over the ring-polymer thermal distribution. To obtain the time-correlation functions and spectra shown in Fig. 2.6 for the OBABO and BCOCB integration schemes, 12-nanosecond T-RPMD trajec- tories were simulated for a ring polymer with 64 beads and timesteps ranging from 0.2 to 1.4 fs, using the same friction schedule as described for Figs. 2.4 and 2.5.

Along each trajectory, the velocities of all degrees of freedom in the system were drawn anew from the Maxwell-Boltzmann distribution every 20 picoseconds; the autocorrelation function was evaluated out to 2 picoseconds by averaging over stag- gered windows of that time-length within every 20-picosecond trajectory segment;

and exponential-decay extrapolation was used to extend the autocorrelation function before evaluating its numerical Fourier transform to obtain the infrared absorption spectrum.

Fig. 2.6(a-b) present the dipole autocorrelation functions obtained using the OBABO and BCOCB integrators with a range of timesteps. For the OBABO integrator, the calculated correlation function is qualitatively incorrect for timesteps as large as 0.8 fs. For the BCOCB integrator, the resulting correlations functions are far more robust

with respect to timestep. Although modest differences are seen in the exponential tail of the correlation function, the dynamics on vibrational timescales (see inset) is largely unchanged as the timestep is varied from 0.2 fs to 1.4 fs. Fig. 2.6(c) further emphasizes this point by showing the absorption spectrum that is obtained from the BCOCB time-correlation functions with the various timesteps. To minimize bias, we avoided any smoothing of the spectra shown in panel c. It is clearly seen that the librational and bending features (below 2500 cm⁻¹) are visually indistinguishable over the entire range of considered timesteps. To clarify the comparison for the stretching region above 3000 cm⁻¹, we smooth the raw spectra in that region by convolution against a Gaussian kernel with a width of 150 cm⁻¹(see inset). Again, the robustness of the simulated spectrum over this span of timesteps is excellent, with the only significant effect due to finite-timestep error being a slight blue-shifting of the OH stretching frequency for the results using a 1.4-fs timestep, which is nearly three times larger than the typical value employed for the OBABO scheme for simulations with 64 beads. Taken together, these results indicate that the BCOCB integrator provides an excellent description of both PIMD statistics and T-RPMD dynamics in realistic molecular systems, substantially improving the accuracy and stability of previously employed numerical integrators.