Hello. My goal is to obtain the potential of mean force in reaction HO2 -> H + O2. I need to change the bond length of H...O for rare events to get PMF. I have set up the input file:

umbrella rxncor nres 500 trig 20 poly 5 name dist min 0.8 max 8.0

But I got the warning message:

***** ERROR ***** Unknown umbrella specified.
*** LEVEL 0 WARNING *** BOMLEV IS -1
**** Warning **** The following extraneous characters
were found while command processing in CHARMM

NRES 500 TRIG 20 POLY 5 NAME DIST MIN 0.8 MAX 8.0

I am not sure why I got this error message. If I comment out this line, I will run the MD simulation without problem. But the bond length of H...O will not reach these rare events I expected by adding this command:

ensemble open unit 21 write formatted name distance.txt
umbrella corr dist unit 21 sele atom main 1 O6 end sele atom main 1 H7 end

I saw that my bond length of H...O is located between 0.91 and 1.09 Angstrom. It means I still need to use the keyword umbrella rxncor to assign the bond length between 0.8 to 8.0 Angstrom.

In my input file, I am sure I have set up the keywords correctly:

rxncor defi p1 point sele atom main 1 O6 end
rxncor defi p2 point sele atom main 1 H7 end
rxncor defi dist distance p1 p2
rxncor set nrxn 1 dist

umbrella ener nres 500 trig 20 poly 5 mine -2000.0 maxe 2000.0 mint 900.0 maxt 1100.0
umbrella init nsim 500 update 50000 equi 5000 thresh 10.0 agin 5.0 temp 1000.0 wuni 18 ucun 19

Anyone can help figure out why? Thank you very much.