Hi Fei,
If I understand you correctly, you are looking for the middle-reference-frame, which is available from file auxiliary.par after running analyze on a duplex struture. An example would be as follows:
find_pair bdl084.pdb stdout | analyze
Check file auxiliary.par, the section you are looking for is:
*****************************************************************************************
Local middle reference frames
Ox Oy Oz Xx Xy Xz Yx Yy Yz Zx Zy Zz
1 CG/CG 16.84 25.38 24.41 -0.90 0.43 -0.02 0.40 0.82 -0.41 -0.16 -0.38 -0.91
2 GC/GC 16.36 24.39 21.11 -0.46 0.84 -0.28 0.86 0.36 -0.36 -0.20 -0.40 -0.89
3 CG/CG 16.17 23.28 17.92 0.11 0.90 -0.41 0.98 -0.17 -0.12 -0.18 -0.39 -0.90
4 GA/TC 15.72 21.96 15.03 0.62 0.71 -0.33 0.77 -0.63 0.07 -0.16 -0.30 -0.94
5 AA/TT 15.07 21.01 11.89 0.96 0.19 -0.20 0.24 -0.94 0.26 -0.14 -0.30 -0.94
6 AT/AT 14.73 20.48 8.65 0.92 -0.39 -0.03 -0.36 -0.87 0.33 -0.16 -0.29 -0.94
7 TT/AA 14.44 19.97 5.45 0.57 -0.81 0.13 -0.80 -0.52 0.30 -0.17 -0.27 -0.95
8 TC/GA 14.15 19.36 2.16 -0.05 -0.97 0.25 -0.99 0.09 0.14 -0.16 -0.24 -0.96
9 CG/CG 13.18 18.67 -0.90 -0.59 -0.74 0.31 -0.78 0.62 0.01 -0.20 -0.24 -0.95
10 GC/GC 12.62 18.30 -4.04 -0.93 -0.27 0.23 -0.31 0.94 -0.14 -0.18 -0.20 -0.96
11 CG/CG 12.80 18.15 -7.43 -0.93 0.35 0.11 0.33 0.93 -0.18 -0.16 -0.13 -0.98
*****************************************************************************************
HTH,
Xiangjun