最简单的方法是测量两侧P原子的坐标差
set p1 [atomselect top "lipid and name P and XXX"]
set p2 [atomselect top "lipid and name P and YYY"]
set z1 [lindex [measure center $p1] 2]
set z2 [lindex [measure center $p2] 2]
XXX 和 YYY 是定义两侧P 的选择条件, 根据实际情况设定, 如z 值或segname
z1 和 z2 之差就是膜厚度