P is the phase angle of pseudorotation for five-membered rings.
For ribose and deoxyribose sugars in nucleic
acids
(tau4 +tau1)-(tau3+tau0)
P = ATAN (-------------------------)
2tau2 (sin 36+sin 72)
If tau2 is <0, then P=P+180 degree (Altona & Sundaralingam,
1972).
Ref: Altona, C. & Sundaralingam, M. (1972).
J. Am. Chem. Soc. 94, 8205-8212.