[gmx-users] about command do_dssp

[aixintiankong]

Dear prof.
     i use the the gromacs 4.6.1 on my centos6.4 system. After MD ending, i use the do_dssp -f md.xtc -s md.tpr  -o secondary-structure.xpm -sc secondary-structure.xvg to analyze the secondary structrue of the protein. 
    when i perform the do_dssp and select MainChain , the fatal erros come out as follow;
Fatal error:
DSSP executable (usr/local/bin/dssp) does not exist (use setenv DSSP)
it is means thant  i don't install do_dssp? however, when i perform the " which do_dssp" at the terminal,  the termianl output "/usr/local/gromacs-4.6.1/bin/do_dssp. i don't know where is wrong. should i reset the environment of the do_dssp?
   please help me !