# The change box keyword

## Purpose

• This keyword is used to change the simulation box. The box variables and the atom positions are changed according to the following equations:

$$\left( \begin{array}{ccc} a_x^{\rm new} & b_x^{\rm new} & c_x^{\rm new} \\ a_y^{\rm new} & b_y^{\rm new} & c_y^{\rm new} \\ a_z^{\rm new} & b_z^{\rm new} & c_z^{\rm new} \end{array} \right) = \left( \begin{array}{ccc} \mu_{xx} & \mu_{xy} & \mu_{xz} \\ \mu_{yx} & \mu_{yy} & \mu_{yz} \\ \mu_{zx} & \mu_{zy} & \mu_{zz} \\ \end{array} \right) \left( \begin{array}{ccc} a_x^{\rm old} & b_x^{\rm old} & c_x^{\rm old} \\ a_y^{\rm old} & b_y^{\rm old} & c_y^{\rm old} \\ a_z^{\rm old} & b_z^{\rm old} & c_z^{\rm old} \end{array} \right);$$ $$\left( \begin{array}{c} x^{\rm new}_i \\ y^{\rm new}_i \\ z^{\rm new}_i \end{array} \right) = \left( \begin{array}{ccc} \mu_{xx} & \mu_{xy} & \mu_{xz} \\ \mu_{yx} & \mu_{yy} & \mu_{yz} \\ \mu_{zx} & \mu_{zy} & \mu_{zz} \\ \end{array} \right) \left( \begin{array}{c} x_i^{\rm old} \\ y_i^{\rm old} \\ z_i^{\rm old} \end{array} \right).$$

• The deformation matrix $\mu_{\alpha\beta}$ will be specified by the parameters of this keyword, as we detail below.

## Grammar

• This keyword accepts 1 or 3 or 6 parameters.
• In the case of 1 parameter $\delta$ (in units of Angstrom),
change_box delta


we have $$\left( \begin{array}{ccc} \mu_{xx} & \mu_{xy} & \mu_{xz} \\ \mu_{yx} & \mu_{yy} & \mu_{yz} \\ \mu_{zx} & \mu_{zy} & \mu_{zz} \\ \end{array} \right) = \left( \begin{array}{ccc} \frac{a_x^{\rm old} + \delta}{a_x^{\rm old}} & 0 & 0 \\ 0 & \frac{b_y^{\rm old} + \delta}{b_y^{\rm old}} & 0 \\ 0 & 0 & \frac{c_z^{\rm old} + \delta}{c_z^{\rm old}} \\ \end{array} \right)$$

• In the case of 3 parameters, $\delta_{xx}$ (in units of Angstrom), $\delta_{yy}$ (in units of Angstrom), and $\delta_{zz}$ (in units of Angstrom),
change_box delta_xx delta_yy delta_zz


we have $$\left( \begin{array}{ccc} \mu_{xx} & \mu_{xy} & \mu_{xz} \\ \mu_{yx} & \mu_{yy} & \mu_{yz} \\ \mu_{zx} & \mu_{zy} & \mu_{zz} \\ \end{array} \right) = \left( \begin{array}{ccc} \frac{a_x^{\rm old} + \delta_{xx}}{a_x^{\rm old}} & 0 & 0 \\ 0 & \frac{b_y^{\rm old} + \delta_{yy}}{b_y^{\rm old}} & 0 \\ 0 & 0 & \frac{c_z^{\rm old} + \delta_{zz}}{c_z^{\rm old}} \\ \end{array} \right)$$

• In the case of 6 parameters (the box type must be triclinic), $\delta_{xx}$ (in units of Angstrom), $\delta_{yy}$ (in units of Angstrom), $\delta_{zz}$ (in units of Angstrom), $\epsilon_{yz}$ (dimensionless strain), $\epsilon_{xz}$ (dimensionless strain), and $\epsilon_{xy}$ (dimensionless strain),
change_box delta_xx delta_yy delta_zz epsilon_yz epsilon_xz epsilon_xy


we have $$\left( \begin{array}{ccc} \mu_{xx} & \mu_{xy} & \mu_{xz} \\ \mu_{yx} & \mu_{yy} & \mu_{yz} \\ \mu_{zx} & \mu_{zy} & \mu_{zz} \\ \end{array} \right) = \left( \begin{array}{ccc} \frac{a_x^{\rm old} + \delta_{xx}}{a_x^{\rm old}} & \epsilon_{xy} & \epsilon_{xz} \\ \epsilon_{yx} & \frac{b_y^{\rm old} + \delta_{yy}}{b_y^{\rm old}} & \epsilon_{yz} \\ \epsilon_{zx} & \epsilon_{zy} & \frac{c_z^{\rm old} + \delta_{zz}}{c_z^{\rm old}} \\ \end{array} \right)$$