@classmethod
def trigonal(cls, a, alpha):
    """
        Initializes a Box in standard trigonal setting using only trigonal lattice
        parameters.  

        a = b = c, alpha = beta = gamma < 120

        Parameters
        ----------
        - a : float
            The a lattice constant
        - alpha : float
            The alpha lattice angle in degrees

        Returns
        -------
        atomman.Box
        """
    if alpha >= 120.0:
        raise ValueError('trigonal alpha angle must be less than 120 degrees')
    return cls(a=a, b=a, c=a, alpha=alpha, beta=alpha, gamma=alpha)
