How to mix calculation methods for a big sized systems??

Questions
#1

We have calculated the system on the basis of the hybrid basis method. However, it is still very slow. Therefore, we would like to explore it using a mixed calculation method. How is this done? How can the outer system use the force field or dftb or DFT methods and the inner layer use high-precision methods? The input file is as follows:
$molecule
0 1
C 3.32644 1.27182 4.31863
C 3.53844 0.06582 3.60763
C 3.32644 1.27182 5.73963
C 3.53844 0.06582 6.44963
C 3.32644 1.27182 8.58163
C 3.53844 0.06582 7.87063
C 3.32644 1.27182 10.00263
C 3.53844 0.06582 10.71263
C 3.32644 1.27182 12.84463
C 3.53844 0.06582 12.13363
C 3.32644 1.27182 14.26563
C 3.53844 0.06582 14.97563
C 3.32644 1.27182 17.10763
C 3.53844 0.06582 16.39663
C 3.32644 1.27182 18.52863
C 3.53844 0.06582 19.23863
C 3.32644 1.27182 21.37063
C 3.53844 0.06582 20.65963
C 1.77644 3.11882 4.31863
C 2.71444 2.33182 3.60763
C 1.77644 3.11882 5.73963
C 2.71444 2.33182 6.44963
C 1.77644 3.11882 8.58163
C 2.71444 2.33182 7.87063
C 1.77644 3.11882 10.00263
C 2.71444 2.33182 10.71263
C 1.77644 3.11882 12.84463
C 2.71444 2.33182 12.13363
C 1.77644 3.11882 14.26563
C 2.71444 2.33182 14.97563
C 1.77644 3.11882 17.10763
C 2.71444 2.33182 16.39663
C 1.77644 3.11882 18.52863
C 2.71444 2.33182 19.23863
C 1.77644 3.11882 21.37063
C 2.71444 2.33182 20.65963
C -0.59856 3.53782 4.31863
C 0.62544 3.53782 3.60763
C -0.59856 3.53782 5.73963
C 0.62544 3.53782 6.44963
C -0.59856 3.53782 8.58163
C 0.62544 3.53782 7.87063
C -0.59856 3.53782 10.00263
C 0.62544 3.53782 10.71263
C -0.59856 3.53782 12.84463
C 0.62544 3.53782 12.13363
C -0.59856 3.53782 14.26563
C 0.62544 3.53782 14.97563
C -0.59856 3.53782 17.10763
C 0.62544 3.53782 16.39663
C -0.59856 3.53782 18.52863
C 0.62544 3.53782 19.23863
C -0.59856 3.53782 21.37063
C 0.62544 3.53782 20.65963
C -2.68756 2.33182 4.31863
C -1.74956 3.11882 3.60763
C -2.68756 2.33182 5.73963
C -1.74956 3.11882 6.44963
C -2.68756 2.33182 8.58163
C -1.74956 3.11882 7.87063
C -2.68756 2.33182 10.00263
C -1.74956 3.11882 10.71263
C -2.68756 2.33182 12.84463
C -1.74956 3.11882 12.13363
C -2.68756 2.33182 14.26563
C -1.74956 3.11882 14.97563
C -2.68756 2.33182 17.10763
C -1.74956 3.11882 16.39663
C -2.68756 2.33182 18.52863
C -1.74956 3.11882 19.23863
C -2.68756 2.33182 21.37063
C -1.74956 3.11882 20.65963
C -3.51156 0.06582 4.31863
C -3.29956 1.27182 3.60763
C -3.51156 0.06582 5.73963
C -3.29956 1.27182 6.44963
C -3.51156 0.06582 8.58163
C -3.29956 1.27182 7.87063
C -3.51156 0.06582 10.00263
C -3.29956 1.27182 10.71263
C -3.51156 0.06582 12.84463
C -3.29956 1.27182 12.13363
C -3.51156 0.06582 14.26563
C -3.29956 1.27182 14.97563
C -3.51156 0.06582 17.10763
C -3.29956 1.27182 16.39663
C -3.51156 0.06582 18.52863
C -3.29956 1.27182 19.23863
C -3.51156 0.06582 21.37063
C -3.29956 1.27182 20.65963
C -2.68756 -2.20018 4.31863
C -3.29956 -1.14018 3.60763
C -2.68756 -2.20018 5.73963
C -3.29956 -1.14018 6.44963
C -2.68756 -2.20018 8.58163
C -3.29956 -1.14018 7.87063
C -2.68756 -2.20018 10.00263
C -3.29956 -1.14018 10.71263
C -2.68756 -2.20018 12.84463
C -3.29956 -1.14018 12.13363
C -2.68756 -2.20018 14.26563
C -3.29956 -1.14018 14.97563
C -2.68756 -2.20018 17.10763
C -3.29956 -1.14018 16.39663
C -2.68756 -2.20018 18.52863
C -3.29956 -1.14018 19.23863
C -2.68756 -2.20018 21.37063
C -3.29956 -1.14018 20.65963
C -0.59856 -3.40618 4.31863
C -1.74956 -2.98718 3.60763
C -0.59856 -3.40618 5.73963
C -1.74956 -2.98718 6.44963
C -0.59856 -3.40618 8.58163
C -1.74956 -2.98718 7.87063
C -0.59856 -3.40618 10.00263
C -1.74956 -2.98718 10.71263
C -0.59856 -3.40618 12.84463
C -1.74956 -2.98718 12.13363
C -0.59856 -3.40618 14.26563
C -1.74956 -2.98718 14.97563
C -0.59856 -3.40618 17.10763
C -1.74956 -2.98718 16.39663
C -0.59856 -3.40618 18.52863
C -1.74956 -2.98718 19.23863
C -0.59856 -3.40618 21.37063
C -1.74956 -2.98718 20.65963
C 1.77644 -2.98718 4.31863
C 0.62544 -3.40618 3.60763
C 1.77644 -2.98718 5.73963
C 0.62544 -3.40618 6.44963
C 1.77644 -2.98718 8.58163
C 0.62544 -3.40618 7.87063
C 1.77644 -2.98718 10.00263
C 0.62544 -3.40618 10.71263
C 1.77644 -2.98718 12.84463
C 0.62544 -3.40618 12.13363
C 1.77644 -2.98718 14.26563
C 0.62544 -3.40618 14.97563
C 1.77644 -2.98718 17.10763
C 0.62544 -3.40618 16.39663
C 1.77644 -2.98718 18.52863
C 0.62544 -3.40618 19.23863
C 1.77644 -2.98718 21.37063
C 0.62544 -3.40618 20.65963
C 3.32644 -1.14018 4.31863
C 2.71444 -2.20018 3.60763
C 3.32644 -1.14018 5.73963
C 2.71444 -2.20018 6.44963
C 3.32644 -1.14018 8.58163
C 2.71444 -2.20018 7.87063
C 3.32644 -1.14018 10.00263
C 2.71444 -2.20018 10.71263
C 3.32644 -1.14018 12.84463
C 2.71444 -2.20018 12.13363
C 3.32644 -1.14018 14.26563
C 2.71444 -2.20018 14.97563
C 3.32644 -1.14018 17.10763
C 2.71444 -2.20018 16.39663
C 3.32644 -1.14018 18.52863
C 2.71444 -2.20018 19.23863
C 3.32644 -1.14018 21.37063
C 2.71444 -2.20018 20.65963
O 0.01401 0.06577 13.86933
H 0.50428 -0.68201 14.21097
H 0.06455 -0.01061 12.90717
O 0.01343 0.06592 10.99991
H -0.89548 0.04856 10.69456
H 0.37173 0.90209 10.69644
$end

$rem
METHOD wb97x
basis mixed
jobtype opt
SCF_ALGORITHM DIIS
THRESH 14
SCF_CONVERGENCE 8
SCF_MAX_CYCLES 100000
MAX_DIIS_CYCLES=1000000
GEOM_OPT_MAX_CYCLES 100000
XC_GRID 000099000590
SYM_IGNORE true
SYMMETRY false
MEM_TOTAL =60000
MEM_STATIC = 2000
$end

$opt
FIXED
1 X Y Z
2 X Y Z
3 X Y Z
4 X Y Z
5 X Y Z
6 X Y Z
7 X Y Z
8 X Y Z
9 X Y Z
10 X Y Z
11 X Y Z
12 X Y Z
13 X Y Z
14 X Y Z
15 X Y Z
16 X Y Z
17 X Y Z
18 X Y Z
19 X Y Z
20 X Y Z
21 X Y Z
22 X Y Z
23 X Y Z
24 X Y Z
25 X Y Z
26 X Y Z
27 X Y Z
28 X Y Z
29 X Y Z
30 X Y Z
31 X Y Z
32 X Y Z
33 X Y Z
34 X Y Z
35 X Y Z
36 X Y Z
37 X Y Z
38 X Y Z
39 X Y Z
40 X Y Z
41 X Y Z
42 X Y Z
43 X Y Z
44 X Y Z
45 X Y Z
46 X Y Z
47 X Y Z
48 X Y Z
49 X Y Z
50 X Y Z
51 X Y Z
52 X Y Z
53 X Y Z
54 X Y Z
55 X Y Z
56 X Y Z
57 X Y Z
58 X Y Z
59 X Y Z
60 X Y Z
61 X Y Z
62 X Y Z
63 X Y Z
64 X Y Z
65 X Y Z
66 X Y Z
67 X Y Z
68 X Y Z
69 X Y Z
70 X Y Z
71 X Y Z
72 X Y Z
73 X Y Z
74 X Y Z
75 X Y Z
76 X Y Z
77 X Y Z
78 X Y Z
79 X Y Z
80 X Y Z
81 X Y Z
82 X Y Z
83 X Y Z
84 X Y Z
85 X Y Z
86 X Y Z
87 X Y Z
88 X Y Z
89 X Y Z
90 X Y Z
91 X Y Z
92 X Y Z
93 X Y Z
94 X Y Z
95 X Y Z
96 X Y Z
97 X Y Z
98 X Y Z
99 X Y Z
100 X Y Z
101 X Y Z
102 X Y Z
103 X Y Z
104 X Y Z
105 X Y Z
106 X Y Z
107 X Y Z
108 X Y Z
109 X Y Z
110 X Y Z
111 X Y Z
112 X Y Z
113 X Y Z
114 X Y Z
115 X Y Z
116 X Y Z
117 X Y Z
118 X Y Z
119 X Y Z
120 X Y Z
121 X Y Z
122 X Y Z
123 X Y Z
124 X Y Z
125 X Y Z
126 X Y Z
127 X Y Z
128 X Y Z
129 X Y Z
130 X Y Z
131 X Y Z
132 X Y Z
133 X Y Z
134 X Y Z
135 X Y Z
136 X Y Z
137 X Y Z
138 X Y Z
139 X Y Z
140 X Y Z
141 X Y Z
142 X Y Z
143 X Y Z
144 X Y Z
145 X Y Z
146 X Y Z
147 X Y Z
148 X Y Z
149 X Y Z
150 X Y Z
151 X Y Z
152 X Y Z
153 X Y Z
154 X Y Z
155 X Y Z
156 X Y Z
157 X Y Z
158 X Y Z
159 X Y Z
160 X Y Z
161 X Y Z
162 X Y Z
ENDFIXED
$end

$basis
C 1
sto-3g

C 2
sto-3g

C 3
sto-3g

C 4
sto-3g

C 5
sto-3g

C 6
sto-3g

C 7
sto-3g

C 8
sto-3g

C 9
sto-3g

C 10
sto-3g

C 11
sto-3g

C 12
sto-3g

C 13
sto-3g

C 14
sto-3g

C 15
sto-3g

C 16
sto-3g

C 17
sto-3g

C 18
sto-3g

C 19
sto-3g

C 20
sto-3g

C 21
sto-3g

C 22
sto-3g

C 23
sto-3g

C 24
sto-3g

C 25
sto-3g

C 26
sto-3g

C 27
sto-3g

C 28
sto-3g

C 29
sto-3g

C 30
sto-3g

C 31
sto-3g

C 32
sto-3g

C 33
sto-3g

C 34
sto-3g

C 35
sto-3g

C 36
sto-3g

C 37
sto-3g

C 38
sto-3g

C 39
sto-3g

C 40
sto-3g

C 41
sto-3g

C 42
sto-3g

C 43
sto-3g

C 44
sto-3g

C 45
sto-3g

C 46
sto-3g

C 47
sto-3g

C 48
sto-3g

C 49
sto-3g

C 50
sto-3g

C 51
sto-3g

C 52
sto-3g

C 53
sto-3g

C 54
sto-3g

C 55
sto-3g

C 56
sto-3g

C 57
sto-3g

C 58
sto-3g

C 59
sto-3g

C 60
sto-3g

C 61
sto-3g

C 62
sto-3g

C 63
sto-3g

C 64
sto-3g

C 65
sto-3g

C 66
sto-3g

C 67
sto-3g

C 68
sto-3g

C 69
sto-3g

C 70
sto-3g

C 71
sto-3g

C 72
sto-3g

C 73
sto-3g

C 74
sto-3g

C 75
sto-3g

C 76
sto-3g

C 77
sto-3g

C 78
sto-3g

C 79
sto-3g

C 80
sto-3g

C 81
sto-3g

C 82
sto-3g

C 83
sto-3g

C 84
sto-3g

C 85
sto-3g

C 86
sto-3g

C 87
sto-3g

C 88
sto-3g

C 89
sto-3g

C 90
sto-3g

C 91
sto-3g

C 92
sto-3g

C 93
sto-3g

C 94
sto-3g

C 95
sto-3g

C 96
sto-3g

C 97
sto-3g

C 98
sto-3g

C 99
sto-3g

C 100
sto-3g

C 101
sto-3g

C 102
sto-3g

C 103
sto-3g

C 104
sto-3g

C 105
sto-3g

C 106
sto-3g

C 107
sto-3g

C 108
sto-3g

C 109
sto-3g

C 110
sto-3g

C 111
sto-3g

C 112
sto-3g

C 113
sto-3g

C 114
sto-3g

C 115
sto-3g

C 116
sto-3g

C 117
sto-3g

C 118
sto-3g

C 119
sto-3g

C 120
sto-3g

C 121
sto-3g

C 122
sto-3g

C 123
sto-3g

C 124
sto-3g

C 125
sto-3g

C 126
sto-3g

C 127
sto-3g

C 128
sto-3g

C 129
sto-3g

C 130
sto-3g

C 131
sto-3g

C 132
sto-3g

C 133
sto-3g

C 134
sto-3g

C 135
sto-3g

C 136
sto-3g

C 137
sto-3g

C 138
sto-3g

C 139
sto-3g

C 140
sto-3g

C 141
sto-3g

C 142
sto-3g

C 143
sto-3g

C 144
sto-3g

C 145
sto-3g

C 146
sto-3g

C 147
sto-3g

C 148
sto-3g

C 149
sto-3g

C 150
sto-3g

C 151
sto-3g

C 152
sto-3g

C 153
sto-3g

C 154
sto-3g

C 155
sto-3g

C 156
sto-3g

C 157
sto-3g

C 158
sto-3g

C 159
sto-3g

C 160
sto-3g

C 161
sto-3g

C 162
sto-3g

O 163
aug-cc-pVTZ

H 164
aug-cc-pVTZ

H 165
aug-cc-pVTZ

O 166
aug-cc-pVTZ

H 167
aug-cc-pVTZ

H 168
aug-cc-pVTZ

$end
waitting for your reply.

#2

Here are some suggestions/comments but you are not really listening to what I have been telling you.
(1) With a minimal basis, this job is 842 basis functions - not trivially small, but not altogether large for a geometry optimization. (You do not need aug-cc-pVTZ for your waters with DFT; 6-31+G* would work just fine, and there’s no reason to crank the DFT grid way up as you’ve done.)

(2) If you are going to use a minimal basis set, you might as well use something like HF-3c/MINIX, which is a parameterized semi-empirical method that uses a minimal basis.

(3) Q-Chem is a fast code so the time you are seeing is just what it takes. (How long it takes for me is about 2 min for energy and 20 sec for gradient, on 40 cores, using HF-3c.) As I’ve told you before, inclusion of all those fixed-atom constraints is likely to lead to an optimization that requires a LOT of steps.

(4) You could try QM/MM with the nanotube treated QM but that may or may not capture the physics that you are trying to describe, and setting up/running QM/MM calculations is definitely not black box. I suggest that you seek help from a more experienced computational chemist because it doesn’t sound like you’ve done this sort of thing before.

(5) Another suggestion is that perhaps you could only constrain the terminal C atoms at the ends of the nanotube, then optimize the full remaining geometry. Those constraints will keep the ends from “reconstructing”, but will be a small number of constraints so that maybe the optimization doesn’t wander around for so many cycles. The way that fixed-atom constraints work is that those entries in the gradient are simply zeroed out (in Cartesian coordinates), and when you are zeroing out so many entries that seems like a recipe not to converge.

#3

Thank you for your reply. According to your suggestions, i will consider this problem.

#4

FYI, on a 40-core node with a minimal basis (e.g., HF-3c method), I can take 50 optimization steps in about 3 hr wall time. Does seem to be converging very slowly. The issue with fixed-atom constraints is that it forces the optimizer to use Cartesian coordinates, and this generally means hundreds of optimization steps.