Units

The standard units in RASPA from which all other units are derived are:

Quantity	Symbol	Unit	Value
Length	\(l\)	Ångstrom	\(10^{-10}\) m
Temperature	\(T\)	Kelvin	K
Mass	\(m\)	Atomic mass	\(1.6605402 \times 10^{-27}\) kg
Time	\(t\)	Pico seconds	\(10^{-12}\) s
Charge	\(q\)	Atomic charge	\(1.60217733 \times 10^{-19}\) C/particle

Some examples of derived units:

Quantity	Symbol	Units	Conversion value
Energy	\(U\)	\(J = \text{mass} \times \text{length}^2 / \text{time}^2\)	\(1.66054 \times 10^{-23}\) (= \(10\) J/mol)
Pressure	\(p\)	\(\text{Pa} = \text{mass} / (\text{length} \times \text{time}^2)\)	\(1.66054 \times 10^7\)
Diffusion constant	\(D\)	\(D = \text{length}^2 / \text{time}\)	\(1 \times 10^{-8}\)
Force	\(f\)	\(f = \text{length} / \text{time}^2\)	\(1.66054 \times 10^{-13}\)
...	...	...	...

A pressure input of 10 Pascal in the input file is converted to 'internal units' by dividing by \(1.66054 \times 10^7\). In the output, any internal pressure is printed, multiplied by \(1.66054 \times 10^7\). It is not necessary to convert units besides input and output, with a few exceptions.

One of the exceptions is the Coulombic conversion factor: $$\frac{q_i q_j}{4\pi \epsilon_0}=\frac{\text{charge}^2}{4 \pi \times \text{electric constant} \times \text{length} \times \text{energy}} = 138935.4834964017$$ where the electric constant is \(8.8541878176 \times 10^{-12}\) in units of \(\text{C}^2/(\text{N} \cdot \text{m}^2)\). This factor is needed to convert the electrostatic energy to the internal units at every evaluation.

The Boltzmann constant \(k_B\) is: $$k_B = \text{Boltzmann constant}/\text{energy} = 0.8314464919$$ with the Boltzmann constant as \(1.380650324 \times 10^{-23}\) in units of J/K, and \(k_B = 0.8314464919\) in internal units.

• Numbering is based on the C-convention, i.e. starting from zero.
• Files in the current directory always have preference. Sometimes one would like to try various parameters for force field fitting, for example. In order to avoid making a lot of directories for each force field, it is more convenient to have the force_field.def file in the current directory.