In the Charmm force field linear virtual site are use for halogens to model halogen bonding. By design the distance between vsite particle and halogen atom is fixed at CL-v: 0.016 nm, BR-v: 0.019 nm, and I-v: 0.022 nm The present virtual\_site2 does not support this since the position of the vsite is relative to two atoms and hence the distance can fluctuate. Here the proposal is make an additional virtual\_site (type 2) with fixed distance to the halogen. *(from redmine: issue id 2451, created on 2018-03-14 by dspoel, closed on 2018-09-24)* * Changesets: * Revision b766e6ebfa539173b4b3580ca5356533a27a8caa by Berk Hess on 2019-09-16T10:47:21Z: ``` Add virtual site type 2FD Fixes #2451 Change-Id: Ide9dcd829284567010435ed5cfb55194aed12dcf ```