Nose-Hoover-chain thermostat

The standard Nose Hoover suffers from well known issues, such as the ergodicity violation in the case of simple harmonic oscillator. As proposed by Martyna and Klein, these problems can be solved by using multiple Nose Hoover thermostats connected in a chain. Although the underlining dynamics is non-Hamiltonian, the corresponding equations of motion conserve the following energy term:

$\mathcal{H'} = \mathcal{H}(\bold{r},\bold{p}) + \sum\limits_{j=1}^{M} \frac{p_{\eta_j}^2}{2Q_j} + 3Nk_{B} T \eta_1 + k_{B} T \sum\limits_{j=2}^{M} \eta_j$

where $\mathcal{H}(\bold{r},\bold{p})$ is the Hamiltonian of the physical system, $M$ and $N$ are the numbers of thermostats and atoms in the cell, respectively, and $\eta_{j}$, $p_{\eta_j}$, and $Q_{j}$ are the position, momentum, and mass-like parameter associated with the thermostat $j$. Just like the total energy in NVE ensemble,$\mathcal{H'}$ is valuable for diagnostics purposes. Indeed, a significant drift in $\mathcal{H'}$ indicate that the corresponding computational setting is suboptimal. Typical reasons for this behavior involve noisy forces (e.g., because of a poor SCF convergence) and/or a too large integration step (defined via POTIM).

The number of thermostats is controlled by the flag NHC_NCHAINS. Typically, this flag is set to a value between 1 and 5, the maximal allowed value is 20. In the special case of NHC_NCHAINS=0, the thermostat is switched off, leading to a MD in microcanonical ensemble. Another special case of NHC_NCHAINS=1 corresponds to the standard Nose-Hoover thermostat.

The only thermostat parameter is NHC_PERIOD, corresponding to a characteristic time scale ($\tau$) of the system expressed in time steps. This variable is used to setup the mass-like variables via the relations:

$Q_1 = 3 N k_{B} T \tau^2$

$Q_j = k_{B} T \tau^2; \; \; \; j=2,\dots,M$

NHC_NRESPA

NHC_NS