update gromacs/wham2d example for dialanine

examples/data/plumed.dat

@@ -0,0 +1,13 @@
+# set up two variables for Phi and Psi dihedral angles 
+phi: TORSION ATOMS=5,7,9,15
+psi: TORSION ATOMS=7,9,15,17
+#
+# Impose an umbrella potential on CV 1 and CV 2
+# with a spring constant of 500 kjoule/mol
+# at fixed points on the Ramachandran plot
+#
+restraint-phi: RESTRAINT ARG=phi KAPPA=100 AT=LAMBDA1
+restraint-psi: RESTRAINT ARG=psi KAPPA=100 AT=LAMBDA2
+
+# monitor the two variables and the bias potential from the two restraints
+PRINT STRIDE=10 ARG=phi,psi,restraint-phi.bias,restraint-psi.bias FILE=COLVAR UPDATE_FROM=50