def identify_env(G):
    """
    Identify the environment of the central node (index 0) of a graph
    
    Input:  - G     Input graph
    
    Output: - env   environment of the central node in G
    """
    
    # Initialize array of neighbouring elements
    nei_elems = []
    # Identify all nodes bonded to the central node
    for e in G.edges:
        
        if 0 in e:
            # Get neighbour atom
            if e[0] == 0:
                i = e[1]
            else:
                i = e[0]
            # Update array of neighbouring elements
            nei_elems.append(G.nodes[i]["elem"])
    
    # Return the environment in string format,
    #   with neighbours sorted alphabetically
    return "-".join(sorted(nei_elems))