Calculate the energy difference of MM and QMMM

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Hello, everyone,
      I am trying to calculate the energy difference of a system using classical force field and QMMM. 

Blow is my MM input file:

 title "single point energy QMMM calculation ethanol in water"
start etl

 #permanent_dir ./perm
scratch_dir /tmp/nw

 prepare
source etl0.pdb
new_top new_seq
new_rst
center
orient
solvate box 3.0
update lists
ignore
write etl_ref.rst
write etl_ref.pdb
end
task prepare

 md
system etl_ref
end

 task md energy

Next is my QMMM single point input file:
 title "single point energy QMMM calculation ethanol in water"
start etl

 #permanent_dir ./perm
scratch_dir /tmp/nw

 prepare
source etl0.pdb
new_top new_seq
new_rst
modify atom 1:_C1 quantum
modify atom 1:2H1 quantum
modify atom 1:3H1 quantum
modify atom 1:4H1 quantum
center
orient
solvate box 3.0
update lists
ignore
write etl_ref.rst
write etl_ref.pdb
end
task prepare

 md
system etl_ref
end

 basis
* library "6-31G"
end
dft
xc b3lyp
end

 qmmm
link_atoms hydrogen
end

 task qmmm dft energy

And the "etl0.pdb" file is as:
ATOM 1 O etl 1 1.201 -0.271 -0.000 1.00 0.00 O
ATOM 2 H etl 1 1.995 0.329 -0.000 1.00 0.00 H
ATOM 3 C1 etl 1 -1.180 -0.393 0.000 1.00 0.00 C
ATOM 4 2H1 etl 1 -2.128 0.155 -0.000 1.00 0.00 H
ATOM 5 3H1 etl 1 -1.130 -1.030 0.887 1.00 0.00 H
ATOM 6 4H1 etl 1 -1.130 -1.030 -0.887 1.00 0.00 H
ATOM 7 C2 etl 1 0.006 0.573 0.000 1.00 0.00 C
ATOM 8 2H2 etl 1 -0.042 1.220 0.890 1.00 0.00 H
ATOM 9 3H2 etl 1 -0.042 1.220 -0.890 1.00 0.00 H
END

  How to get the energy difference of system using QM/MM method and purely MM?  I am massed by the output file.
Thanks very much!
Yours'
Xiao
Edited On 3:14:40 PM PDT - Thu, Jun 9th 2011 by Xiaosy567

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You will already have QM/MM energy from your simulation. You need to run a separate calculation to get purely classical energy. You will have to use classical MD module for that, not QM/MM


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