Help menu for run_xDH_using_Gaussian.py
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  #Filename:  run_xDH_using_Gaussian.py
  #Author  :  Igor Ying Zhang, Xin Xu
  #Version :  1.0(20210516)
  #Usage   :  run_xDH_using_Gaussian.py [options] file
  #           Try 'run_xDH_using_Gaussian.py --help' for more information
  #Install :  1) Dependence: python 3.x;
  #                          Gaussian xx (xx=03, 09, or 16).
  #           2) Excute '[xDH4Gau]/config'.
  #           3) Edit '[xDH4Gau]/Gxx_Environment (xx=03, 09, or 16)
  #              to setup the proper interface of the Gaussian package(s)
  #              that is(are) installed in your OS.
  #               
  #Purpose :  1) Run XYG3-type doubly hybrid (xDH) calculations using the
  #              Gaussian package.
  #              See the paper for more details about the xDH methods: 
  #                 Zhang, I. Y.; Xu, X. "Exploring the Limits of the XYG3-Type 
  #                 Doubly Hybrid Approximations for the Main-Group Chemistry: 
  #                 The XDH@B3LYP Model." 
  #                 J. Phys. Chem. Lett. 2021, 12, 2638–2644. 
  #           2) Share the same input format as the standard Gaussian jobs,
  #              such that the xDH methods (like XYG3) are invoked as
  #              the standard DFT methods (like B3LYP) in case insensitivity.
  #Example::
  #           The B3LYP calculation use the standard Gaussian input as:
  #           #=========================================================#
  #           #    %nproc=8                                             #
  #           #    #p b3lyp/6-311+G(3df,2p) scf=tight                   #
  #           #                                                         #
  #           #    CH4 B3LYP calculation                                #
  #           #                                                         #
  #           #    0 1                                                  #
  #           #    c                                                    #
  #           #    h   1 hc2                                            #
  #           #    h   1 hc3        2 hch3                              #
  #           #    h   1 hc4        3 hch4         2 dih4               # 
  #           #    h   1 hc5        4 hch5         3 dih5               # 
  #           #                                                         # 
  #           #    hc2         1.086000                                 # 
  #           #    hc3         1.086046                                 #
  #           #    hch3        109.470                                  #
  #           #    hc4         1.170267                                 #
  #           #    hch4        104.203                                  #
  #           #    dih4        127.944                                  #
  #           #    hc5         1.170393                                 #
  #           #    hch5        107.454                                  #
  #           #    dih5       -115.152                                  #
  #           #                                                         # 
  #           #=========================================================#
  #         
  #           To perform the XYG3 calculation, simply replace the keyword
  #           of "b3lyp" by "xyg3". Results are collected in the output 
  #           file with the postfix of '.xDH'
  #           #=========================================================#
  #           #    %nproc=8                                             #
  #           #    #p xyg3/6-311+G(3df,2p) scf=tight                    #
  #           #                                                         #
  #           #    CH4 XYG3 calculation                                 #
  #           #                                                         #
  #           #    0 1                                                  #
  #           #    c                                                    #
  #           #    h   1 hc2                                            #
  #           #    h   1 hc3        2 hch3                              #
  #           #    h   1 hc4        3 hch4         2 dih4               # 
  #           #    h   1 hc5        4 hch5         3 dih5               # 
  #           #                                                         # 
  #           #    hc2         1.086000                                 # 
  #           #    hc3         1.086046                                 #
  #           #    hch3        109.470                                  #
  #           #    hc4         1.170267                                 #
  #           #    hch4        104.203                                  #
  #           #    dih4        127.944                                  #
  #           #    hc5         1.170393                                 #
  #           #    hch5        107.454                                  #
  #           #    dih5       -115.152                                  #
  #           #                                                         # 
  #           #=========================================================#

==> More tests could be found in the direction of "Tests"