CHARMM Development Project Forums User Discussion & Questions Parameter Set Discussion 1-4 vdw interaction

I notice that in Charmm parameter files some atoms have two sets of vdw parameters - one for the 'normal' vdw interaction and the other for 1-4 vdw interaction. For example:

C 0.000000 -0.110000 2.000000 ! ALLOW PEP POL ARO
! NMA pure solvent, adm jr., 3/3/93

and
CT1 0.000000 -0.020000 2.275000 0.000000 -0.010000 1.900000 ! ALLOW ALI
! isobutane pure solvent properties, adm jr, 2/3/92

My question is this: if an atom does not contain the second set of parameters (such as C as shown above), does Charmm still include the 1-4 interactions for an atom pair that contain atom C?

yes

If so, which set of parameters is it using? Is it the first set?

If the second set is missing the values from the first set are used as 1-4 parameters for that atomtype.