  A new ab initio interaction potential for the electronic ground state of the
argon dimer has been developed. The new potential uses previously calculated
accurate Born--Oppenheimer potential while significantly improving the
description of relativistic effects by including the two-electron Darwin and
orbit-orbit corrections. Moreover, retardation of the electromagnetic
interactions is taken into account in the long-range part of the potential and
leading-order quantum electrodynamics correction is calculated. Spectroscopic
properties of the argon dimer such as positions of vibrational levels,
bond-dissociation energy, and rotational and centrifugal-distortion constants
are reported. We show that the inclusion of the two-electron relativistic terms
results in a destabilization of the previously discovered weakly bound ninth
vibrational state. Finally, thermophysical properties of the argon gas
including pressure and acoustic virial coefficients, as well as transport
properties -- viscosity and thermal conductivity -- are evaluated using the new
potential. For the thermophysical properties, the obtained ab initio values are
somewhat less accurate than the most recent experimental results. However, the
opposite is true for the transport properties, where the theoretical results
calculated in this work have significantly smaller uncertainties than the data
derived from measurements.
