CONS DIHE used to be the only choice, and required a loop over the helix residues, e.g. for residues 22-45 of segment A--
set k 22 ! FIRST RES
label rhlx
calc m = @K + 1
cons dihe A @K N A @K CA A @K C A @M N force 100. min -47 ! psi
cons dihe A @K C A @M N A @M CA A @M C force 100. min -57 ! phi
incr k by 1
if k lt 44 goto rhlx
However, a few years ago CONS HARM was enhanced with a couple new keywords, notably the BESTFIT option--
cons harm bestfit mass force 1. sele resid 22:45 .and. -
( type N .or. type CA .or. type C ) end
The force values differ so much because the CONS DIHE force units are force/radian, while the CONS HARM units are (mass weighted) force/A^2. Note that CONS HARM without a 3rd keyword reverts to CONS HARM ABSOlute, which restrains the atom coordinates; the BESTFIT option restrains the RMS coord difference after alignment via rotation and translation, and thus restrains the shape of the group of selected atoms.
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