[gmx-users] calculating gbsa parameters for metal ion

[Javaria ashraf]

Hello and greetingsi am simulating my protein in implicit solvent. the protein has ca2+ ion in it. the gbsa.itp file doesn't have parameters for any ion.how can i add parameters of my ion?As mentioned in tutorial.....[ implicit_genborn_params ];Atomtype sar st pi gbr hct
Values in columns 1-3 are not currently used. Column 4 contains the atomic van der Waals radii, which are used in computing the Born radii. 
the van der Waals radii of ca2+ is 0.231? is it right? or how can i calculate the hct?thank you