  A unified molecular field theory (MFT) is presented that applies to both
collinear and planar noncollinear Heisenberg antiferromagnets (AFs) on the same
footing. The spins in the system are assumed to be identical and
crystallographically equivalent. This formulation allows calculations of the
anisotropic magnetic susceptibility chi versus temperature T below the AF
ordering temperature TN to be carried out for arbitrary Heisenberg exchange
interactions J{ij} between arbitrary neighbors j of a given spin i without
recourse to magnetic sublattices. The Weiss temperature theta_p in the
Curie-Weiss law is written in terms of the J{ij} values and TN in terms of the
J{ij} values and an assumed AF structure. Other magnetic and thermal properties
are then expressed in terms of quantities easily accessible from experiment as
laws of corresponding states for a given spin S. For collinear ordering these
properties are the reduced temperature t = T/TN, the ratio f = theta_p/TN and
S. For planar noncollinear helical or cycloidal ordering, an additional
parameter is the wavevector of the helix or cycloid. The MFT is also applicable
to AFs with other AF structures. The MFT predicts that chi(T <= TN) of
noncollinear 120 degree spin structures on triangular lattices is isotropic and
independent of S and T and thus clarifies the origin of this universally
observed behavior. The high-field magnetization and heat capacity for fields
applied perpendicular to the ordering axis (collinear AFs) and ordering plane
(planar noncollinear AFs) are also calculated and expressed for both types of
AF structures as laws of corresponding states for a given S, and the reduced
perpendicular field versus reduced temperature phase diagram is constructed.
