def get_negative_pos(positions):
    """Returns the minimum negative atomic positions vector"""
    xnmin = 0.0
    ynmin = 0.0
    znmin = 0.0
    for line in positions:
        line_s = line.split()
        xnmin = min( xnmin, float(line_s[1]) ) 
        ynmin = min( ynmin, float(line_s[2]) ) 
        znmin = min( znmin, float(line_s[3]) ) 
    return xnmin, ynmin, znmin