# Force_field_params and LJ potential

We want to use oniom method to calculate aqueos water inside nanotube,in which the C atomtype and other parameters of nanotube seems not be defined. I look up previous papers, showing that the L-J parameter between two oxygen atoms as σOO = 0.315 nm and EOO = 0.1521 kcal/mol.Moreover, the L-J parameters between two carbon atoms are σCC = 0.355nm and ECC = 0.293 kJ/mol, the L-J parameters between carbon and oxygen atoms are σCO = 0.335nm and ECO = 0.103184 kcal/mol. However, we only define the carbon atomtype,as follow:
\$force_field_params
NumAtomTypes 2
AtomType -1 0 3.55 0.293
Then, how are the L-J parameters between the carbon and oxygen atoms and between the two oxygen atoms determined? And do the water clusters in the nanotubes need to be defined parameters or is the SPC/E model applied?

It is difficult to answer this question without having a an input file. Is it possible for you to construct a small example (maybe not a nanotube)? What force field are you trying to modify?

Hello! I would like to ask you how to calculate ONIOM and frequency at the same time. We found that the nanotubes are not confined and that the vibrational modes only show the vibrations of the nanotubes, rather than water clusters. The specific input file is as follows:
\$rem
JOBTYPE = freq
METHOD = pbe0
BASIS = aug-cc-pVDZ
QM_MM_INTERFACE = oniom
FORCE_FIELD = oplsaa
USER_CONNECT = true
MOLDEN_FORMAT = true
GEOM_OPT_DRIVER = OPTIMIZE
\$end

\$molecule
0 1
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O -0.0182943933 -0.0430859761 13.8578421176 -2 192 193 0 0
H 0.4900721544 -0.7891732947 14.1836396762 -3 191 0 0 0
H 0.0263133514 -0.1094770141 12.8921784052 -3 191 0 0 0
O -0.0312980400 -0.0291403502 10.9800266285 -2 195 196 0 0
H -0.9421396244 -0.0565901924 10.6727157455 -3 194 0 0 0
H 0.3134314733 0.8143434176 10.6725735886 -3 194 0 0 0
\$end

\$opt
FIXED
1 XYZ
2 XYZ
3 XYZ
4 XYZ
5 XYZ
6 XYZ
7 XYZ
8 XYZ
9 XYZ
10 XYZ
11 XYZ
12 XYZ
13 XYZ
14 XYZ
15 XYZ
16 XYZ
17 XYZ
18 XYZ
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21 XYZ
22 XYZ
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24 XYZ
25 XYZ
26 XYZ
27 XYZ
28 XYZ
29 XYZ
30 XYZ
31 XYZ
32 XYZ
33 XYZ
34 XYZ
35 XYZ
36 XYZ
37 XYZ
38 XYZ
39 XYZ
40 XYZ
41 XYZ
42 XYZ
43 XYZ
44 XYZ
45 XYZ
46 XYZ
47 XYZ
48 XYZ
49 XYZ
50 XYZ
51 XYZ
52 XYZ
53 XYZ
54 XYZ
55 XYZ
56 XYZ
57 XYZ
58 XYZ
59 XYZ
60 XYZ
61 XYZ
62 XYZ
63 XYZ
64 XYZ
65 XYZ
66 XYZ
67 XYZ
68 XYZ
69 XYZ
70 XYZ
71 XYZ
72 XYZ
73 XYZ
74 XYZ
75 XYZ
76 XYZ
77 XYZ
78 XYZ
79 XYZ
80 XYZ
81 XYZ
82 XYZ
83 XYZ
84 XYZ
85 XYZ
86 XYZ
87 XYZ
88 XYZ
89 XYZ
90 XYZ
91 XYZ
92 XYZ
93 XYZ
94 XYZ
95 XYZ
96 XYZ
97 XYZ
98 XYZ
99 XYZ
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101 XYZ
102 XYZ
103 XYZ
104 XYZ
105 XYZ
106 XYZ
107 XYZ
108 XYZ
109 XYZ
110 XYZ
111 XYZ
112 XYZ
113 XYZ
114 XYZ
115 XYZ
116 XYZ
117 XYZ
118 XYZ
119 XYZ
120 XYZ
121 XYZ
122 XYZ
123 XYZ
124 XYZ
125 XYZ
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127 XYZ
128 XYZ
129 XYZ
130 XYZ
131 XYZ
132 XYZ
133 XYZ
134 XYZ
135 XYZ
136 XYZ
137 XYZ
138 XYZ
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142 XYZ
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168 XYZ
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175 XYZ
176 XYZ
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179 XYZ
180 XYZ
181 XYZ
182 XYZ
183 XYZ
184 XYZ
185 XYZ
186 XYZ
187 XYZ
188 XYZ
189 XYZ
190 XYZ
ENDFIXED
\$end

\$qm_atoms
191 192 193 194 195 196
\$end

what is your user-defined FF parameters section for this job?