Improper section for parameterization

From: Theodora Teddy (teddygirl1001_at_gmail.com)
Date: Tue Nov 21 2017 - 07:20:59 CST

Hello,
I am doing parameterization of a small molecule, using the topology and the
parameter file from CHARMM27. I have a question about the IMPROPER section
in the parameter file. What does X means, found in some lines like the
following?

CPB X X C 90.0000 0 0.0000 ! ALLOW HEM

! Heme (6-liganded): substituents (KK 05/13/91)

CT2 X X CPB 90.0000 0 0.0000 ! ALLOW HEM

! Heme (6-liganded): substituents (KK 05/13/91)

Thank you for your time.

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