ML LCOUPLE

ML_FF_LCOUPLE_MB = [logical]
Default: ML_FF_LCOUPLE_MB = .FALSE. 

Description: This tag specifies whether coupling parameters are used for the calculation of chemical potentials is used or not within the machine learning force field method.

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In thermodynamic integration a coupling parameter [math]\displaystyle{ \lambda }[/math] is introduced to the Hamiltonian to smoothly switch between a "non-interacting" reference state and a "fully-interacting" state. The change of the free energy along this path is written as

[math]\displaystyle{ \Delta \mu = \int\limits_{0}^{1} \langle \frac{dH(\lambda)}{d\lambda} \rangle_{\lambda} d\lambda. }[/math]

Using machine learning force fields the Hamiltonian can be written as

[math]\displaystyle{ H (\lambda) = \sum\limits_{i=1}^{N_{a}} \frac{|\mathbf{p}_{i}|^2}{2m_{i}} + \sum\limits_{i \notin M} U_{i}(\lambda) + \lambda \sum\limits_{i \in M} U_{i}(\lambda) + \sum\limits_{i}^{N_{a}} U_{i,\mathbf{atom}}. }[/math]

where [math]\displaystyle{ N_{a} }[/math] denotes the number of atoms and [math]\displaystyle{ U_{i,\mathbf{atom}} }[/math] is an atomic reference energy for a single non interacting atom. The first term in the equation describes the potential energy and the second and third term describe the potential energy of an atom [math]\displaystyle{ i }[/math]. The index [math]\displaystyle{ M }[/math] denotes the atoms whose interaction is controlled by a coupling parameter. The interaction of the atoms are controlled by scaling the contributions to the atom density via the coupling parameter

[math]\displaystyle{ \rho (\mathbf{r},\lambda) = \sum\limits_{j\notin M} f_{\mathrm{cut}} \left( \left| \mathbf{r}_{j} - \mathbf{r}_{i} \right| \right) g \left[ \mathbf{r} - \left( \mathbf{r}_{j} - \mathbf{r}_{i} \right) \right] + \lambda \sum\limits_{j\in M} f_{\mathrm{cut}} \left( \left| \mathbf{r}_{j} - \mathbf{r}_{i} \right| \right) g \left[ \mathbf{r} - \left( \mathbf{r}_{j} - \mathbf{r}_{i} \right) \right]. }[/math]

Further details on the implementation can be found in reference .

For thermodynamic integration the following parameters have to be set:

• ML_FF_ISTART=2.
• ML_FF_LCOUPLE_MB=.TRUE..
• The number of atoms for which a coupling parameter is introduced ([math]\displaystyle{ i \notin M }[/math]): ML_FF_NATOM_COUPLED_MB.
• The list of atom indices that for that the coupling parameter is applied in the interaction: ML_FF_ICOUPLE_MB.
• The strength of the coupling parameter [math]\displaystyle{ lambda }[/math] between 0 and 1: ML_FF_RCOUPLE_MB.

The derivative of the hamiltonian with respect to the coupling constant [math]\displaystyle{ dH/d\lambda }[/math] is written out at every MD step to the ML_LOGFILE. A sample output should look like the following

dH/dRCOUPLE (eV):     0.893558

Related Tags and Sections

ML_FF_LMLFF, ML_FF_NATOM_COUPLED_MB, ML_FF_ICOUPLE_MB, ML_FF_RCOUPLE_MB

Examples that use this tag

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