take into account the quantum effects of atomic nuclei in calculations based on the quantum thermal bath method

doc/gpumd/input_parameters/ensemble_qtb.rst

@@ -0,0 +1,109 @@
+.. _kw_ensemble_qtb:
+.. index::
+   single: nvt_qtb (keyword in run.in)
+   single: npt_qtb (keyword in run.in)
+   single: Quantum Thermal Bath
+
+:attr:`ensemble` (QTB)
+======================
+
+The variants of the :attr:`ensemble` keyword described on this page implement the Quantum Thermal Bath (QTB) method [Dammak2009]_.
+The QTB thermostat is a Langevin-type thermostat that uses colored noise with a quantum Bose-Einstein energy spectrum instead of classical white noise.
+This allows approximate inclusion of nuclear quantum effects (zero-point energy and quantum heat capacity) in otherwise classical MD simulations.
+
+The :attr:`npt_qtb` variant combines the QTB thermostat with the Parrinello-Rahman (MTTK) barostat [Martyna1994]_, equivalent to LAMMPS ``fix nph`` + ``fix qtb``.
+
+
+Syntax
+------
+
+:attr:`nvt_qtb`
+^^^^^^^^^^^^^^^
+
+Run an NVT simulation with the QTB thermostat::
+
+    ensemble nvt_qtb <T_1> <T_2> <T_coup> [f_max <value>] [N_f <value>]
+
+* :attr:`<T_1>` and :attr:`<T_2>`: Initial and final target temperature (K). The target temperature varies linearly during the run.
+* :attr:`<T_coup>`: Thermostat coupling parameter (in units of timestep). Controls the friction coefficient: :math:`\gamma = 1 / (\text{T\_coup} \times dt)`.
+* :attr:`f_max`: (Optional, default 200) Maximum frequency of the QTB filter in ps\ :sup:`-1`. Should be larger than the highest phonon frequency in the system.
+* :attr:`N_f`: (Optional, default 100) Number of frequency points in the filter. The filter uses :math:`2 N_f` points total.
+
+:attr:`npt_qtb`
+^^^^^^^^^^^^^^^
+
+Run an NPT simulation with the QTB thermostat and Parrinello-Rahman (MTTK) barostat::
+
+    ensemble npt_qtb <direction> <p_1> <p_2> temp <T_1> <T_2> tperiod <tau_T> pperiod <tau_p> [f_max <value>] [N_f <value>]
+
+Pressure control parameters:
+
+* :attr:`<direction>`: One or more of ``iso``, ``aniso``, ``tri``, ``x``, ``y``, ``z``. Same syntax as :ref:`npt_mttk <mttk>`.
+* :attr:`<p_1>` and :attr:`<p_2>`: Initial and final target pressure (GPa).
+
+Temperature and coupling parameters:
+
+* :attr:`temp <T_1> <T_2>`: Initial and final target temperature (K).
+* :attr:`tperiod <tau_T>`: QTB thermostat coupling period (in units of timestep). Controls friction: :math:`\gamma = 1 / (\text{tperiod} \times dt)`.
+* :attr:`pperiod <tau_p>`: Barostat coupling period (in units of timestep, must be :math:`\geq 200`).
+
+QTB-specific optional parameters (same as :attr:`nvt_qtb`):
+
+* :attr:`f_max`: Maximum frequency (ps\ :sup:`-1`, default 200).
+* :attr:`N_f`: Number of frequency points (default 100).
+
+
+Examples
+--------
+
+NVT-QTB
+^^^^^^^^
+
+.. code-block:: rst
+
+    ensemble nvt_qtb 300 300 100
+
+Run at 300 K with QTB thermostat. The coupling parameter is 100 timesteps.
+
+.. code-block:: rst
+
+    ensemble nvt_qtb 300 300 100 f_max 150 N_f 200
+
+Same as above but with custom filter parameters.
+
+NPT-QTB
+^^^^^^^^
+
+.. code-block:: rst
+
+    ensemble npt_qtb iso 0 0 temp 300 300 tperiod 100 pperiod 1000
+
+Run at 300 K and 0 GPa with isotropic pressure control.
+
+.. code-block:: rst
+
+    ensemble npt_qtb aniso 0 0 temp 300 300 tperiod 100 pperiod 1000 f_max 200 N_f 100
+
+Anisotropic pressure control with explicit QTB parameters.
+
+.. code-block:: rst
+
+    ensemble npt_qtb x 5 5 y 0 0 z 0 0 temp 300 300 tperiod 100 pperiod 1000
+
+Apply 5 GPa along x and 0 GPa along y and z.
+
+
+Notes
+-----
+
+* The QTB method generates colored noise whose power spectrum matches the quantum energy distribution :math:`E(\omega) = \hbar\omega[\frac{1}{2} + n_{BE}(\omega, T)]`, where :math:`n_{BE}` is the Bose-Einstein distribution.
+* The kinetic temperature reported in ``thermo.out`` will be higher than the target temperature due to zero-point energy contributions. This is expected behavior.
+* For liquid water at 300 K, the kinetic temperature is typically around 1000-1100 K.
+* The :attr:`npt_qtb` ensemble uses the MTTK (Martyna-Tuckerman-Tobias-Klein) integrator for pressure control, which is the same as :ref:`npt_mttk <mttk>` but with the Nosé-Hoover chain thermostat replaced by the QTB thermostat.
+* The :attr:`f_max` parameter should be set larger than the highest phonon frequency in the system. For water, 200 ps\ :sup:`-1` is sufficient.
+
+
+References
+----------
+
+.. [Dammak2009] H. Dammak, Y. Chalopin, M. Laroche, M. Hayoun, and J.-J. Greffet, *Quantum Thermal Bath for Molecular Dynamics Simulation*, Phys. Rev. Lett. **103**, 190601 (2009).

doc/gpumd/input_parameters/index.rst

@@ -23,6 +23,7 @@ Below you can find a listing of keywords for the ``run.in`` input file.
    ensemble
    ensemble_standard
    ensemble_mttk
+   ensemble_qtb
    ensemble_heat
    ensemble_pimd
    ensemble_ti_liquid