# Virial Basic Layout

When the Virial component of the application is started, it looks like this:

The left side of the display is used for numerical data input (*B* at different temperatures). At the bottom are
three sliders and input fields which are used to adjust the basic parameters of the LJ potential ($\varepsilon$ (“epsilon”) and $\sigma$ (“sigma”) according to Eq. (4)) and the quadrupole moment *Q*. At the center of the interface is the
Function Display where the analytical B(T ) calculated from Eq. (7) and additional quadrupole terms is
displayed together with the data points.
Data points that are entered in the data input field are automatically added as points in the Function
Display. If any of the sliders is moved the function B(T) will change accordingly. By adjusting the potential
parameters one can bring the calculated B(T) curve to agree with the experimental data, as shown in the
screenshot below:

This way, the LJ parameters and the quadrupole moment can be obtained from experimentally determined
virial coefficients *B*. These potential parameters can be used for subsequent Molecular Dynamics (MD) or
Monte Carlo (MC) simulations.
The unit for $\sigma$ is $\text{\AA} = 10^{-10}m$. The parameter $\varepsilon$ corresponds to the depth of the LJ potential well
and therefore measures the interaction energy at the molecular level. The usual energy unit in SI is Joule
(J) but the value for the interaction between two molecules is tiny compared to a Joule. The Boltzmann
constant *k* is of “atomic dimension”; therefore $\varepsilon$ is often given in units of *kT* and one simply specifies a
temperature *T* instead. Therefore, the module reads the $\varepsilon$ energy value in Kelvin. The dimension of the
quadrupole moment is $charge \cdot length^2$. Again, SI units are adapted to macroscopic objects and lead to very
small numerical values at the molecular scale. Here, the quadrupole moment is given in units of $debye \cdot \text{\AA}$
with 1 debye = $1 \cdot 10^{-18}$ esu·cm. The unit esu is a unit of electrical charge and comes from the old CGS unit system (Centimeter–Gram–Second). The corresponding SI unit is $C \cdot m^2$.