计算两个小分子之间的RMSD，可以用来判断两个构象的接近程度。

第一步：安装Anaconda

Win或者Linux系统下Anaconda安装，不赘述，网上很多教程。

第二步：安装RDKit

通过conda安装RDKit

conda install -c rdkit rdkit

python isoRMSD.py mol1.pdb mol2.pdb rmsd.txt

#!/usr/bin/python
'''
calculates RMSD differences between 2 conformation with different atom names.
@author: JC <yangjincai@nibs.ac.cn>
'''
import os
import sys
import math
 
# rdkit imports
from rdkit import Chem
from rdkit.Chem import AllChem
from rdkit.Chem.AllChem import AlignMol
 
def GetBestRMSD(probe,ref,refConfId=-1,probeConfId=-1,maps=None):
  """ Returns the optimal RMS for aligning two molecules, taking
  symmetry into account. As a side-effect, the probe molecule is
  left in the aligned state.
  Arguments:
    - ref: the reference molecule
    - probe: the molecule to be aligned to the reference
    - refConfId: (optional) reference conformation to use
    - probeConfId: (optional) probe conformation to use
    - maps: (optional) a list of lists of (probeAtomId,refAtomId)
      tuples with the atom-atom mappings of the two molecules.
      If not provided, these will be generated using a substructure
      search.
  Note: 
  This function will attempt to align all permutations of matching atom
  orders in both molecules, for some molecules it will lead to 'combinatorial 
  explosion' especially if hydrogens are present.  
  Use 'rdkit.Chem.AllChem.AlignMol' to align molecules without changing the
  atom order.
  """
  # When mapping the coordinate of probe will changed!!!
  ref.pos = orginXYZ(ref)
  probe.pos = orginXYZ(probe)
 
  if not maps:
    matches = ref.GetSubstructMatches(probe,uniquify=False)
    if not matches:
      raise ValueError('mol %s does not match mol %s'%(ref.GetProp('_Name'),
                                                       probe.GetProp('_Name')))
    if len(matches) > 1e6: 
      warnings.warn("{} matches detected for molecule {}, this may lead to a performance slowdown.".format(len(matches), probe.GetProp('_Name')))
    maps = [list(enumerate(match)) for match in matches]
  bestRMS=1000.0
  bestRMSD = 1000.0
  for amap in maps:
    rms=AlignMol(probe,ref,probeConfId,refConfId,atomMap=amap)
    rmsd = RMSD(probe,ref,amap)
    if rmsd<bestRMSD:
      bestRMSD = rmsd
    if rms<bestRMS:
      bestRMS=rms
      bestMap = amap
    
 
  # finally repeate the best alignment :
  if bestMap != amap:
    AlignMol(probe,ref,probeConfId,refConfId,atomMap=bestMap)
    
  return bestRMS, bestRMSD
 
# Map is probe -> ref
# [(1:3),(2:5),...,(10,1)]
def RMSD(probe,ref,amap):
  rmsd = 0.0
  # print(amap)
  atomNum = ref.GetNumAtoms() + 0.0
  for (pi,ri) in amap:
    posp = probe.pos[pi]
    posf = ref.pos[ri]
    rmsd += dist_2(posp,posf)
  rmsd = math.sqrt(rmsd/atomNum)
  return rmsd
 
def dist_2(atoma_xyz, atomb_xyz):
  dis2 = 0.0
  for i, j  in zip(atoma_xyz,atomb_xyz):
    dis2 += (i -j)**2
  return dis2
 
def orginXYZ(mol):
  mol_pos={}
  for i in range(0,mol.GetNumAtoms()):
    pos = mol.GetConformer().GetAtomPosition(i)
    mol_pos[i] = pos
  return mol_pos
 
if  __name__ == "__main__":
  usage="""
  isoRMSD.py will output two RMSD, one is fitted, another is no fit.
  Not fit RMSD mean no change in molecules coordinates. 
  
  Usage:python isoRMSD.py mol1.pdb mol2.pdb rmsd.txt
  """
  if len(sys.argv) < 4:
    print(usage)
    sys.exit()
  
  ref = Chem.MolFromPDBFile(sys.argv[1])
  probe = Chem.MolFromPDBFile(sys.argv[2])
 
 
  # here, rms is Fitted, rmsd is NOT Fit!!!
  rms,rmsd = GetBestRMSD(probe,ref)
 
  print("\nBest_RMSD: %.3f\nBest_Not_Fit_RMSD: %.3f\n"%(rms,rmsd))
  out = open(sys.argv[3],"w")
  out.write("Best_RMSD: %.3f\nBest_Not_Fit_RMSD: %.3f\n"%(rms,rmsd))
  out.close()

参考链接：

https://github.com/0ut0fcontrol/isoRMSD