Euler prove the Pentagonal number theorem, which is about the denominator of the generating function for the partition function $p(n)$, that is $\phi(q)=\prod_{k=1}^\infty(1-q^k)=\sum_{n=-\infty}^\infty(-1)^nq^{(3n^2-n)/2}$
My question is: can we determine the a closed form of $\phi(q)$ for special rational values of $q$? (e.g., $\phi(1/2)$ or $\phi(1/3)$)