Documented power spectrum to gmx velacc

This option was saved by the bell and my nostalgia for
power spectra. The usual recourse is that we get rid of
functionality that nobody takes the time to document.

Fixes #2019.

Change-Id: I09fc5367a59300e2fca780125893fad5c1d72b81

diff --git a/src/gromacs/gmxana/gmx_velacc.cpp b/src/gromacs/gmxana/gmx_velacc.cpp
index 902b9617cf1dfa504045e67f8f08bcb8b3c8b550..a13e358a98d5253b6fdaa1c90bc8bb355c6231ca 100644 (file)
--- a/src/gromacs/gmxana/gmx_velacc.cpp
+++ b/src/gromacs/gmxana/gmx_velacc.cpp
@@ -185,6 +185,9 @@ int gmx_velacc(int argc, char *argv[])
         "With option [TT]-mol[tt] the velocity autocorrelation function of",
         "molecules is calculated. In this case the index group should consist",
         "of molecule numbers instead of atom numbers.[PAR]",
+        "By using option [TT]-os[tt] you can also extract the estimated",
+        "(vibrational) power spectrum, which is the Fourier transform of the",
+        "velocity autocorrelation function."
         "Be sure that your trajectory contains frames with velocity information",
         "(i.e. [TT]nstvout[tt] was set in your original [REF].mdp[ref] file),",
         "and that the time interval between data collection points is",