c1: READ FILE=colvar.in VALUES=cv1
c2: READ FILE=colvar.in VALUES=cv2

ff: COLLECT_FRAMES ARG=c1,c2 
pca: PCA USE_OUTPUT_DATA_FROM=ff METRIC=EUCLIDEAN NLOW_DIM=2 
OUTPUT_PCA_PROJECTION USE_OUTPUT_DATA_FROM=pca FILE=pca-comp.pdb FMT=%8.4f 
