# [AMBER] Question about QM/MM simulation in vacuum connecting Amber and Gaussian 09 package

From: 王婷婷 <ttwang.mail.dlut.edu.cn>
Date: Tue, 9 Apr 2019 08:36:31 +0800 (GMT+08:00)

Recently, I performed QM/MD simulation of two molecules in vacuum with igb=6. I set the system temperature at 300K, however, the average temperature of the simulation, which I found at the end of the simulation output file, is quiet abnormal.

I performed two simulations in vacuum. The first is in pure vacuum, and the average temperature of system is about 470K. The second is also in vacuum and extra electric field was added (dielc=78.5), a more dramatic change in temperature was observed and the average temperature of this system is about 2000K !

I’m quiet confused about this unusual phenomenon. I send this e-mail for your help, I want to know why the system temperature varied so huge. Is it normal and can be used for further analysis?

And If there is something wrong, how can I put the temperature stable.

Here is the input：
【vacuum/e=78.5】

300K constant temp QMMMMD
&cntrl
imin = 0, irest=0,ntx=1,
ntb = 0,
igb= 6 ,
dielc= 78.5 ,
cut = 9.0,ntx=1,
tempi = 298.0, temp0 = 298.0,
ntt = 3, gamma_ln = 0,
nstlim = 20000, dt = 0.001,
ntpr = 20, ntwx = 20 , ntwr = 20
ntc=2, ntf=2,nmropt=1, ifqnt=1,
/
&wt type='DUMPFREQ', istep1=100 /
&wt type='END' /
DISANG=N-C-5.0.RST
DUMPAVE=N-C-15.0-rstN.out
/
&qmmm
qmcharge=0,
qm_theory='EXTERN',
qmshake=0,
qmcut=12.0,
qm_ewald=0
/

&gau
method = 'B3LYP'
basis = '6-31+G**'
charge = 0
scf_conv=6
&end

【pure vacuum】

300K constant temp QMMMMD
&cntrl
imin = 0, irest=0,ntx=1,
ntb = 0,
igb=6,
cut = 9.0,ntx=1,
tempi = 298.0, temp0 = 298.0,
ntt = 3, gamma_ln = 1.0,
nstlim = 10000, dt = 0.001,
ntpr = 20, ntwx = 20 , ntwr = 20
ntc=2, ntf=2,nmropt=1, ifqnt=1,
/
&wt type='DUMPFREQ', istep1=100 /
&wt type='END' /
DISANG=N-C-5.0.RST
DUMPAVE=N-C-15.0-rstN.out
/
&qmmm
qmcharge=0,
qm_theory='EXTERN',
qmshake=0,
qmcut=12.0,
qm_ewald=0
/

&gau
method = 'B3LYP'
basis = '6-31+G**'
charge = 0
scf_conv=6
&end

------------------------------------------------------------------------------------

地址：辽宁省大连市甘井子区凌工路2号 环境楼
邮编：116024
电话：183-4223-5107
Tingting Wang
Key Laboratory of Industrial Ecology and Environmental Engineering (MOE) , CHINA
School of Environmental Science and Technology, Dalian University of Technology Dalian, CHINA

116024,P. R.
Phone Number: +86 183-4223-5107

E-mail: ttwang.mail.dlut.edu.cn
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Received on Mon Apr 08 2019 - 18:00:03 PDT
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