def Gmat2A(G):
    """Extract A from reciprocal metric tensor (G)

    :param G: reciprocal maetric tensor (3x3 numpy array
    :return: A = [G11,G22,G33,2*G12,2*G13,2*G23]

    """
    return [G[0][0], G[1][1], G[2][2], 2. * G[0][1], 2. * G[0][2], 2. * G[1][2]]