Step 2 - Set up a Forcefield
To simulate any system we need to supply suitable parameters to describe the interactions between atoms, both internally within the same species and externally between atoms of the same (and different) species. The ff tab is always available, and summarises the atom types used over all defined species and the resulting pair potentials, as well as master intramolecular definitions (which we aren’t using in this example). You can also set other important options, such as the range of the generated pair potentials and the truncation schemes.
We’ll now describe the interactions for your argon species by taking the terms from one of Dissolve’s built-in forcefields.
Go to the Ar species tab
By selecting the tab containing the Argon species you are making it ‘active’, and so it will be the target of any actions triggered from the Species menu.
Species ⇨ Add Forcefield Terms…
From the available forcefields choose
OPLSAA2005/NobleGasesand click
Here we choose how to assign atom types to the species - the default option of Determine atom types for all atoms uses connectivity descriptions in the forcefield to try and automatically choose which type to use. Leave this option selected and click
There will be no conflicts between the proposed atom types and existing ones defined in the main simulation (since there aren’t any) so click to complete the wizard
For more complicated (molecular) species the wizard also handles how intra-molecular terms are generated. You also may have to deal with potential naming conflicts with existing terms in the main simulation, which the Add Forcefield Terms wizard will help you resolve.
If you now return to the Forcefield
tab you will see in the Atom Types section that we have an atom type for our argon atom which contains the necessary interaction parameters. The short range type (SR Form in the table) specifies the functional form of the van der Waals interaction parameters for each atom type, which in our case is LJGeometric indicating that the parameters reflect a standard Lennard-Jones potential utilising geometric combination rules. Those parameters (epsilon and sigma) are shown in the SR Parameters column and describe how our argon atoms will interact with each other in the simulation.
Atom types are strictly associated with a chemical element, and can only be applied to atoms of the same element.
By default, Dissolve will generate all the necessary pair potentials for the current set of atom types automatically - these are listed in the Pair Potentials section where you can also select individual potentials and visualise them in the associated plot.
Before we move on we need to set an option related to the charges to use. Along with van der Waals parameters atomic charges are the second important component in describing interactions between atoms. In Dissolve these charges can either be taken from the atom types and included in the tabulated pair potentials, or located on species atoms and calculated analytically. Dissolve automatically detects the most appropriate scheme but is wary of systems or species where there are too many atoms with zero charge, as is the case here. As such, we must force a choice - in the Charge Source group at the top-right of the ff tab deselect the Choose Automatically option and enable the Force Choice option below it. The actual charge source (Atom Types vs Species Atoms) is unimportant since charges are zero in both, but the default option is faster.