Error creating dihedral type

[rsdefever]

I'm trying to create a DihedralType object and I'm getting an error I don't quite understand:

Code:

DihedralType(
    name="Test dihedral",
    expression="kn * (1 + cos(n * a - a0))",
    parameters={
        "kn": float(kn) * u.K * u.kb,
        "n": int(n) * u.dimensionless,
        "a0": float(a0) * u.degree, 
    }
)

Error:

ValueError: symbol phi is not in expression's free symbols. Cannot use an independent variable which doesn't exist in the expression's free symbols {a, kn, n, a0}

[umesh-timalsina]

@rsdefever. This is related to #509, where you specified the parameters for this DihderalType but didn't explictly specify the independent variable and it was assigned the default value which is phi.

[rsdefever]

Thanks @umesh-timalsina, I didn't realize I needed to specify the independent variables. Easy enough 👍 .