Commit e1337fc7 authored by Etienne Mangaud's avatar Etienne Mangaud
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......@@ -30,10 +30,395 @@ Module **PIM_qtb** generates trajectories based on several classical and semi-c
These trajectories can be used to sample initial conditions for intramolecular vibrational-energy redistribution (IVR) dynamics.
Applications of the module
__________________________
This module has been used to calculate time correlation function of an anharmonic oscillator parameterized on a hydroxyle bond OH and to compute pair correlation functions of a 13 atoms' neon cluster described by a Lennard-Jones potential.
Description of the module
=========================
The module implements various methods based on Langevin dynamics to
sample initial conditions for IVR or to directly exploit the generated
trajectories. The methods implemented are: classical Langevin dynamics,
Quantum Thermal Bath (QTB) and two variants of adaptive QTB (adQTB-r and
adQTB-f).
Classical Langevin dynamics
---------------------------
Classical Langevin dynamics is described by a stochastic differential
equation :
.. math:: m \dot v = -\nabla U -m \gamma v + F(t)
:label: eqLGV
where :math:`-\nabla U` is the vector forces exerted on the sets of
atoms, :math:`m` the mass of atoms, :math:`\gamma` the damping term
determining the strength of a viscous force and :math:`F(t)` a
stochastic noise. The stochastic noise is gaussian and delta-correlated.
To ensure the classical fluctuation-dissipation theorem, one might write
its autocorrelation function spectra as :
.. math:: C_{FF}(\omega)= \int \limits_{-\infty}^{+\infty} dt \langle F(t) F(t+\tau) \rangle e^{-i \omega t} = 2m \gamma k_B T
where :math:`k_B` is the Boltzmann constant and :math:`T` the
temperature.
Quantum Thermal Bath (QTB)
--------------------------
Quantum Thermal Bath is an approximate semi-classical method
[Dam]_ consisting in modifying the
stochastic noise in eq. :eq:`eqLGV` in order to mimic the energy
distribution of a set of quantum harmonic oscillators. In QTB dynamics,
stochastic noise is no longer delta-correlated but is colored with the
following form :
.. math:: C_{FF}(\omega)=2m \gamma \theta(\omega,T)
:label: eqQTB
with
.. math:: \theta(\omega,T) = \hbar \omega \left[\frac{1}{2}+\frac{1}{e^{\hbar \beta \omega}-1}\right]
where :math:`\beta = \frac{1}{k_BT}` and :math:`2 \pi \hbar` is Planck
constant. :math:`\theta(\omega,T)` function represents the energy
distribution of a quantum harmonic oscillators of angular frequency
:math:`\omega` at a temperature :math:`T` (Bose function) and notably,
its zero-point energy. This method is known to lead to qualitatively
good results [Bri]_ but as most of the semi-classical methods, it suffers from zero-point energy leakage [Hern]_.
Adaptive Quantum Thermal Bath (adQTB-r/f)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Adaptive Quantum Thermal Bath is a variant of QTB to ensure for each
degree of freedom and each frequency the energy distribution provided by
the quantum fluctuation-dissipation theorem all along the trajectories.
In practice, one minimizes during QTB dynamics a fluctuation-dissipation
relation :math:`\Delta_{FDT}` [Man]_ defined
as:
.. math:: \Delta_{FDT} (\omega) = {\rm{Re}} \left[C_{vF}(\omega)\right] - m \gamma_{r} (\omega) C_{vv} (\omega)
:label: eqDFDT
where :math:`C_{vF}` is the velocity random force cross-correlation
spectrum, :math:`C_{vv}` the velocity autocorrelation spectrum and
:math:`\gamma_{r}` a set of damping coefficients dependent (or not) on
the frequency.
This minimization is carried out by dissymetrizing the system-bath
coefficients between the injected and the extracted energy distribution.
One can do it either by directly modifying the spectrum of the random
noise :math:`F(t)` with frequency dependent damping term
:math:`\gamma_r(\omega)` (adQTB-r variant) or by modifying the memory
dissipative kernel :math:`\gamma_{f} (\omega)` within the framework of a
generalized Langevin equation (adQTB-f variant).
The coefficients :math:`\gamma_r` or :math:`\gamma_f` are slowly
adjusted with a first-order differential equation and a damping term
:math:`A_\gamma` :
.. math:: \frac{d }{dt}\gamma_{r/f} (\omega) \propto A_\gamma \gamma \Delta_{FDT,r/f}(\omega,t)
:label: eqgammadapt
during a “thermalization time” until they reach convergence. Then,
observables are computed by keeping on active the adaptive process.
Further and more precise implementation details can be found in ref.[Man]_.
Two implementations are currently available in PaPIM:
#. Random force adaptive QTB (adQTB-r) In this variant, the dissipation
kernel is left unchanged, i.e. :math:`\gamma_{f}(\omega) = \gamma`
while the random force is modified according to a frequency-dependent
set of damping coefficients :math:`\gamma_r(\omega)` to satisfy
:math:`\Delta_{FDT} = 0` (eq. :eq:`eqDFDT`):
.. math:: C_{FF}(\omega)=2m \gamma_r(\omega) \theta(\omega,T)
:label: eqadQTBr
This method is applicable only if the initial damping coefficient
:math:`\gamma` is large enough to compensate effects of a possible
zero-point energy leakage.
#. Dissipative kernel adaptive QTB (adQTB-f) In this approach, the
random force is not modified (i.e.
:math:`\gamma_{r} (\omega) = \gamma` which remains the same as in QTB
formalism(eq. :eq:`eqQTB`)) but the dissipation term is not only
represented as a mere damping viscous term (:math:`-m \gamma v`) but
as a dissipative memory kernel. It leads to a generalized Langevin
equation:
.. math:: m \dot v = -\nabla U -m \int_0^\infty \ \gamma_f(\tau) v(t-\tau) \ d\tau + F(t)
:label: eqgenlgv
In order to avoid solving with brute force this integro-differential
equation, the dissipative memory kernel is expressed as a sum of
equally spaced (:math:`\Delta \omega`) lorentzian functions of width
:math:`\alpha` :
.. math:: \gamma_f(\omega) = \frac{\Delta \omega}{\pi}\sum_{j=0}^{n_\omega}
\frac{ \gamma_{f,j} }{\alpha + i(\omega-\omega_j)} +\frac{ \gamma_{f,j}}{\alpha + i(\omega+\omega_j)}
:label: eqlorentzgenlgv
The parameter :math:`\gamma_{f,j}` are then modified to satisfy
:math:`\Delta_{FDT} = 0` (eq. :eq:`eqDFDT`). In this method, one should take care of checking results convergence by decreasing the :math:`\alpha` parameter.
Input file
==========
To run PaPIM using one of the Langevin methods, one must set the
parameter *sampling\_type* in the *sampling* section to one of the
following values:
- classical\_langevin
- qtb
- adqtbr
- adqtbf
| In this case the parameters *n\_equilibration\_steps* and
*n\_mc\_steps* are ignored and the section *langevin* is read.
|  
| The section *langevin* must specify the following parameters:
- *dt* : time step of the Langevin dynamics (REAL)
- *lgv\_nsteps* : number of Langevin steps between each IVR sample
(INTEGER)
- *lgv\_nsteps\_therm* : number of thermalization steps (INTEGER)
- *integrator* : integration method (two splitting methods are
currently implemented: BAOAB, ABOBA (see reference
[Lei]_ )) (STRING,
default=“ABOBA”)
- *damping* : base damping coefficient for production runs
(:math:`\gamma` in eq. :eq:eqLGV) (REAL)
- *damping\_therm* : base damping coefficient for thermalization
(:math:`\gamma` in eq. :eq:eqLGV) (REAL)
- *qtb\_frequency\_cutoff* : cutoff frequency for the QTB kernel (REAL)
- *adqtb\_agammas* : (Only for adqtbr and adqtbf) adaptation speed
coefficient for adQTB (:math:`A_\gamma` in eq. :eq:`eqgammadapt`)(REAL)
- *adqtb\_alpha* : (Only for adqtbf) Width of the lorentzian used to
represent the dissipative kernel :math:`\gamma_f(\omega)`
(:math:`\alpha` in eq. :eq:`eqlorentzgenlgv`) (REAL)
- *write\_spectra* : write average random force autocorrelation
function ff, velocity autocorrelation function vv and velocity random
force cross-correlation function vf spectra (LOGICAL, default=.FALSE.)
- *write\_trajectories* : write Langevin trajectories in x,y,z,px,py,pz
format (LOGICAL, default=.FALSE.)
Remark: all physical quantities are specified in Hartree atomic units.
Output files
============
The Langevin module is plugged to the IVR subroutines and thus can
output the same correlation functions as the classical MC sampling.
Additionally, it can write the Langevin trajectories and spectra
obtained directly from them.
Langevin trajectories
---------------------
If the parameter *write\_trajectories* of the *langevin* section of the
input file is set to TRUE, Langevin trajectories are saved. Trajectory
files follow the following format:
::
num_of_atoms
At_symbol(1) X Y Z Px Py Pz
At_symbol(2) X Y Z Px Py Pz
.
.
At_symbol(n) X Y Z Px Py Pz
num_of_atoms
At_symbol(1) X Y Z Px Py Pz
At_symbol(2) X Y Z Px Py Pz
.
.
At_symbol(n) X Y Z Px Py Pz
.
.
.
This corresponds to an extended XYZ format with information on momenta.
It is readable by visualization software such as VMD to display the
trajectories.
The module outputs multiple trajectory files depending on the number of
independent trajectories (blocks) and the number of MPI processes. The
naming follows the rules:
- ``xp.traj.xyz`` for 1 block and 1 process
- ``xp_proci.traj.xyz`` for 1 block and multiple processes
- ``xp_proci_blockj.traj.xyz`` for multiple blocks and processes
QTB analysis files
------------------
In addition to the trajectories, several files can be edited during the
simulations. They are useful to carefully check the convergence of the
adaptive QTB, notably by calculating :math:`\Delta_{FDT}(\omega)` (eq. :eq:`eqDFDT`).
- ``ff_vv_vf_spectra.out`` spectra of random force and velocity
autocorrelation and random force velocity cross-correlation functions
(in atomic units)
:math:`\omega` :math:`C_{FF} (\omega)`
:math:`2m \gamma \theta(\omega,T)` :math:`C_{vv} (\omega)`
:math:`m \gamma C_{vv} (\omega)` :math:`C_{vF} (\omega)`
- ``gamas.out`` (for adQTB-r and adQTB-f only) final set of
:math:`\gamma_{r/f} (\omega)` optimized during the adaptive procedure (in atomic units)
:math:`\omega` :math:`\gamma_{r/f} (\omega)` :math:`\gamma`
Tests on implemented potentials
===============================
OH anharmonic potential
-----------------------
The classical Langevin has been tested on the OH anharmonic potential.
The left panel of Figure :numref:`fig_oh` shows time correlation functions
obtained with IVR using initial conditions sampled from classical
(Boltzmann) Monte Carlo and from classical Langevin. Its right panel
shows the corresponding spectra obtained by Fourier transform.
.. _fig_oh:
.. figure:: oh_lgv_vs_mc_mod.png
Left panel: OH time correlation function using IVR with initial
conditions sampled from MC and from Langevin. Right panel:
corresponding spectra obtained by FFT.
Lennard-Jones :math:`Ne_{13}` cluster
-------------------------------------
A Lennard-Jones potential has been implemented in
``LennardJonesPot.f90`` with the following pair potential:
.. math:: V(r_{ij}) = \sum\limits_{i=1}^{N} \sum\limits_{j>i}^{N}
4 \epsilon \left( \left( \frac{\sigma}{r_{ij}} \right)^{12}
- \left( \frac{\sigma}{r_{ij}} \right)^6 \right)
:label: eqLJ_pot
A confining pair potential (useful in the cases of small clusters) can
be added to eq. :eq:`eqLJ_pot`. A 4th order polynomial is used for
distances greater than a chosen distance :math:`r_{cont}`:
.. math:: V_{conf}(r_{ij}) = \sum \limits_{i=1}^{N} \sum \limits_{j > i}^{N}
\epsilon \left ( r_{ij} - r_{cont} \right)^4
:label: eqLJ_cont
Parameters for this potential are specified in an external text file.
The file name is given in the input file using the parameter
*lennard\_jones\_parameters* in section *system*. The parameters to
specify are:
- *epsil* : depth of the potential well :math:`\epsilon` (in Kelvin)
(eq. :eq:`eqLJ_pot`)
- *sigma* : distance for which the potential cancels :math:`\sigma` (in
Å) (eq. :eq:`eqLJ_pot`)
- *r\_cont* : minimum distance for which a confining potential
:math:`r_{cont}` defined in eq. :eq:`eqLJ_cont` is applied (in Å)
The QTB and both adaptive methods were tested on a Ne13 cluster in order
to reproduce results from reference [Man]_.
The Lennard-Jones parameters which have been used are
:math:`epsil=34.9`, :math:`sigma=2.78` and :math:`r\_cont=10.` 5 runs of
8000 steps with 16000 initial time steps are used with all four methods
(Langevin, QTB, adQTB-r,adQTB-f). Damping term is set to 5.0e-5 atomic
units and adaptive coefficients :math:`A_\gamma` and :math:`\alpha` for
adQTB-f to 5.0e-6 atomic units. Pair correlation function is then
computed from the trajectories output with a Python script
``compute_g2r.py``. Results are shown in figure :numref:`fig_Ne13g2r` and are in
agreement with the ones of ref. [Man]_.
.. _fig_Ne13g2r:
.. figure:: Ne13_g2r.png
Pair correlation function of Ne\ :math:`_{13}` cluster obtained with
Langevin, QTB, adQTB-r and adQTB-f implemented with Langevin module
in PaPIM. Reference curve calculated with Path Integral Molecular
Dynamics (PIMD)
In this particular case, adaptive QTB leads to significantly better
results than both classical Langevin and QTB when comparing them to the
reference results obtained with PIMD (Path Integral Molecular Dynamics).
Implementation
==============
Langevin module is built with the fewest modifications possible in the
main and previous code of PaPIM. The main program of the sampler is in
the file ``langevin.f90``. It is structured in the same fashion as the
existing samplers (``PIM.f90`` and ``ClassMC.f90``) and only provides
the subroutine *langevin\_sampling* to the main program.
Source files
------------
The Langevin module is divided in multiple files:
- ``langevin.f90``: contains the Langevin sampler and links the main
code with the other files of the module
- ``langevin_integrator.f90``: subroutines to integrate Langevin
equations
- ``langevin_analysis.f90``: spectral analysis tools for Langevin and
(ad)QTB trajectories
- ``qtb_random.f90``: generation of QTB colored noise and adaptation
subroutines for adQTB
Other modifications
-------------------
Some other routines have been modified during the implementation of
Langevin module.
- ``PaPIM.f90``: main code ; add calls to Langevin module
- ``GlobType.f90``: add declarations for Langevin
- ``ReadFiles.f90``: read input files
Outlook
=======
The next step in the implementation is to add the Wigner Langevin
dynamics to sample the initial conditions for the IVR.
Furthermore, one should note that the current implementation is
functional with the pre-CP2K version of PaPIM. Thus, some work must be
done to ensure that everything is compatible with the last version of
the code.
Compiling
......@@ -105,6 +490,11 @@ __________
.. [Man] E. Mangaud, S. Huppert, T. Pl'e, P. Depondt, S. Bonella, F. Finocchi, Quantum thermal bath with enforced fluctuation-dissipation theorem for reliable simulations of nuclear quantum effects, Journal of Chemical Theory and Computation, Submitted (2018).
.. [Bri] F. Brieuc, Y. Bronstein, H. Dammak, P. Depondt, F. Finocchi, M. Hayoun, Zero-point energy leakage in quantum thermal bath molecular dynamics simulations, J. Chem. Th. Comput. 12 (2016) 5688–5697.
.. [Hern] J. Hern'andez-Rojas, F. Calvo, E. G. Noya, Applicability of Quantum Thermal Baths to Complex Many-Body Systems with Various Degrees of Anharmonicity, Journal of Chemical Theory and Computation 11 (2015) 861–870.
.. [Lei] B. Leimkuhler, C. Matthews, Rational Construction of Stochastic Numerical Methods for Molecular Sampling, Applied Mathematics Research eXpress (2012).
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