diff --git a/docs/methods/qm_mm/image_charges.md b/docs/methods/qm_mm/image_charges.md new file mode 100644 index 0000000000..3fbc56f39a --- /dev/null +++ b/docs/methods/qm_mm/image_charges.md @@ -0,0 +1,135 @@ +# Image charges + +## Introduction + +The image charge (IC) augmented QM/MM model in CP2K is designed for the simulation of +adsorbate-metal systems. The adsorbate is treated by QM whereas the metallic substrate is described +by classical force fields. + +![](image_charges.png){align=center width=500px} + +**Figure:** Nitrobenzene molecule on Au111. Separation in subsystems for IC-QM/MM. + +The interactions between metal and adsorbate are described at the MM level of theory accounting for +the polarization of metal and adsorbate by an IC approach. The charge distribution $\rho_m$ in the +metal is modeled by a set of Gaussian charges (image charges) centered at the metal atoms, + +$$ + \rho_m(\mathbf{r})= \sum_{a}{c_a g_a(\mathbf{r,R}_a)}, + \label{eq:rhom} +$$ + +where $\mathbf{R}_\mathrm{a}$ is the position of metal atom $a$ and $g_a$ the spherical Gaussian +function located at $a$. The expansion coefficients $c_a$ are unknown and determined in a +self-consistent procedure imposing the constant-potential condition within a metal, i.e. the sum of +$V_H(\mathbf{r})$ generated by the charge density $\rho(\mathbf{r})$ of the molecule and +$V_m(\mathbf{r})$ generated by $\rho_m$ has to be constant within the metal, + +$$ + V_{H}(\mathbf{r})+V_m(\mathbf{r})=\int \frac{\rho(\mathbf{r}')+\rho_m(\mathbf{r}')}{|\mathbf{r}'-\mathbf{r}|} + d\mathbf{r}'=V_0. + \label{eq:const_pot} +$$ + +In this expression $V_0$ is a constant potential that can be different from zero, if an external +potential is applied. The implementation is embedded in the Gaussian and plane waves scheme of CP2K +and thus naturally suited for periodic systems. Details of the theory and implementation are +described in [](#Golze2013). + +## Basic input + +The IC method is specified in the [QMMM](#CP2K_INPUT.FORCE_EVAL.QMMM) section by + +```none +&QMMM + : + : + &IMAGE_CHARGE + MM_ATOM_LIST 1..576 + EXT_POTENTIAL 0.0 + &END IMAGE_CHARGE +&END QMMM +``` + +The keyword [MM_ATOM_LIST](#CP2K_INPUT.FORCE_EVAL.QMMM.IMAGE_CHARGE.MM_ATOM_LIST) defines the list +of MM atoms that carrying an image charge. These are typically all metal atoms. +[EXT_POTENTIAL](#CP2K_INPUT.FORCE_EVAL.QMMM.IMAGE_CHARGE.EXT_POTENTIAL) corresponds to $V_0$ above +and is set to 0.0V by default. Note that the QM and MM box must have the same size for an IC-QM/MM +calculation. + +## Print options + +Detailed energy information and the normalized IC coefficients $q_a$ can be printed out by +[IMAGE_CHARGE_INFO](#CP2K_INPUT.FORCE_EVAL.QMMM.PRINT.IMAGE_CHARGE_INFO). The normalized IC +coefficients are defined as $q_a = c_a\left(\frac{\alpha}{\pi}\right)^{-\frac{3}{2}}$, where +$\alpha$ is the width of the Gaussian. + +```none +&QMMM + : + : + &PRINT + &IMAGE_CHARGE_INFO + &END + &END PRINT +&END QMMM +``` + +## Advanced input options + +Additional keywords that can be set in [IMAGE_CHARGE](#CP2K_INPUT.FORCE_EVAL.QMMM.IMAGE_CHARGE) are: + +```none +&IMAGE_CHARGE + MM_ATOM_LIST 1..576 + EXT_POTENTIAL 0.0 + WIDTH 3.5 + IMAGE_MATRIX_METHOD MME + DETERM_COEFF CALC_MATRIX + RESTART_IMAGE_MATRIX .false. +&END IMAGE_CHARGE +``` + +[WIDTH](#CP2K_INPUT.FORCE_EVAL.QMMM.IMAGE_CHARGE.WIDTH) refers to the width $\alpha$ of the Gaussian +$g_a$, which is the only adjustable parameter in our IC model. The energies and gradients do not +depend on $\alpha$ for values larger than 3.0 Å$^{-2}$. Small values of $\alpha$ correspond to very +broad Gaussians, which will lead to technical artifacts. Extremely large values are not recommended, +too. + +[IMAGE_MATRIX_METHOD](#CP2K_INPUT.FORCE_EVAL.QMMM.IMAGE_CHARGE.IMAGE_MATRIX_METHOD) determines how +the IC matrix $T_{ab}$, Eq. 11 in [](#Golze2013), is calculated. `GPW` corresponds to the algorithm +shown in Fig. 1 in J. Chem. Theory Comput., 9, 5086 (2013), whereas `MME` is an integral scheme that +has been recently implemented and that is significantly faster. + +[DETERM_COEFF](#CP2K_INPUT.FORCE_EVAL.QMMM.IMAGE_CHARGE.DETERM_COEFF) specifies how the expansion +coefficients $c_a$ are determined. With `CALC_MATRIX`, $T_{ab}$ is calculated and the linear set of +equations is solved. `ITERATIVE` avoids calculation of $T_{ab}$ in each SCF step by using an +iterative conjugate gradient scheme. The new integral scheme `MME` evaluates the matrix $T_{ab}$ +very quickly, i.e. the iterative solutions is not required here, thus set always `CALC_MATRIX`. + +[RESTART_IMAGE_MATRIX](#CP2K_INPUT.FORCE_EVAL.QMMM.IMAGE_CHARGE.RESTART_IMAGE_MATRIX) can be used to +restart $T_{ab}$ for an MD simulation if `ITERATIVE` is set. + +```{note} +Setting these additional keywords is typically not required. The default settings are fine. +``` + +## Typical setup + +The typical setup for an IC-QM/MM simulation is as follows + +- adsorbed molecules described by DFT +- metal is constrained or described by an embedded atom model (EAM) +- Interactions between QM and MM: + - Pauli repulsion, dispersion etc. modeled by force fields e.g. Lennard Jones + - electrostatic interaction/induction: IC model + +## Example input files + +The first input example is a single guanine molecule on an Au111 surface using a modified Born-Mayer +potential to describe Pauli repulsion and dispersion between molecule and metal. The second example +is a single water molecule on Pt111. The interactions between water and metal are modeled by the +Siepmann-Sprik potential, see [](#SIEPMANN1995). + +- Guanine@Au111: [Au111_guanine.tar.gz](https://www.cp2k.org/_media/howto:au111_guanine.tar.gz) +- H2O@Pt111: [Pt111_1H2O.tar.gz](https://www.cp2k.org/_media/howto:pt111_1h2o.tar.gz) diff --git a/docs/methods/qm_mm/image_charges.png b/docs/methods/qm_mm/image_charges.png new file mode 100644 index 0000000000..efe3eb8835 Binary files /dev/null and b/docs/methods/qm_mm/image_charges.png differ diff --git a/docs/methods/qm_mm/index.md b/docs/methods/qm_mm/index.md index edb54b5d41..ac0af78147 100644 --- a/docs/methods/qm_mm/index.md +++ b/docs/methods/qm_mm/index.md @@ -9,6 +9,5 @@ builtin gromacs polarizable_ff implicit_solvation -Image charges - +image_charges ```