# Attributes PC, ENERGY and GAMMA of BEAM command not described

There are several possible definitions for PC

`PC = mc^2 \frac{\sqrt{\sum_i p_{x,i}^2 + p_{y,i}^2 + p_{z,i}^2}}{N}`

`PC = mc^2 \frac{\sum_i p_{z,i}}{N}`

Currently the second definition is implemented. This has the advantage that mean longitudinal momentum is well defined and thereby the reference particle has the same velocity as the centroid. The disadvantage is that the kinematic energy of the bunch is to high. The definition for PC should be chosen and the two attributes should be described in the manual for 1.6 and 1.9.