"NaN" or "Infinity" result from "free energy calculation"

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10:22:00 AM PDT - Tue, May 8th 2012

Hello everyone. Nowadays I'm running NWChem to calculate free energy difference between
two similar system configuration.

I tried to see the result by typing

 grep free <output file name>

then I found the message like

 current free energy difference          NaN         NaN

 current free energy difference    -Infinity    Infinity

 current free energy difference    -Infinity    0.385625

 current free energy difference    -Infinity    0.404288

 current free energy difference    -Infinity    0.566782

 current free energy difference    -Infinity    0.682560

 current free energy difference    -Infinity    0.766234

                 ......             ......      ......

   Total free energy difference (kcal/mol): -Infinity

I got "NaN" and "Infinity".

My question is...

What does it mean? What should I do? Why I got such result?
(I think this is not correct result since my system configuration is not so complicated)

There is two column in the output from grep command. (after "current free energy difference" column)
What's the meaning of each column?

Thank you very much in advance.

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