def product_two_gaussians(self, a1, a2, r1, r2):
        '''
            a1 : float
            
                denotes the contraction coefficient of each gaussian or exponent.
        
            r1 : float
            
                denotes the center location of one of the gaussians
                
            r2 : float
            
                denotes the ceter location of second gaussion
                
            return : 
            
                as : float
                
                    sum of the two centers
                    
                ap : float
                
                    product of the two centers
                    
                disp : float
                
                    displacement of two gaussian centers
        '''                
        asum = a1+a2
        aproduct = a1*a2
        dist = r1-r2        
        rp = (a1*r1+a2*r2)/(a1+a2)
        return asum, aproduct, dist, rp

            a1 : float
            
                denotes the contraction coefficient of each gaussian or exponent.
        
            r1 : float
            
                denotes the center location of one of the gaussians
                
            r2 : float
            
                denotes the ceter location of second gaussion
                
            return : 
            
                as : float
                
                    sum of the two centers
                    
                ap : float
                
                    product of the two centers
                    
                disp : float
                
                    displacement of two gaussian centers
        
recipes/Python/578515_Hartree_Fock_SCF/recipe-578515.py