The temperature of the system can be estimated by use of the Equipartition Theorem, with three degrees of freedom for each ion.  Since ionic velocities are generally recorded at each step in the numerical simulation, the average kinetic energy of each ion is easy to calculate.  There exist schemes which attempt to control the temperature of the simulation by, e.g. enforcing each ion to have exactly the kinetic energy predicted by the Equipartition Theorem (Berendsen thermostat) or by allowing the system to exchange energy and momentum with a (more massive) fictitious enclosing system (Nose-Hoover thermostat).
The net force on each ion is generally calculated explicitly at each numerical step.  From this, the stress tensor of the system can be calculated and usually is calculated by the numerical package.  By varying the convergence criteria, one can either seek a lowest energy structure or a structure that produces a desired stress tensor.  Thus, high pressures can be simulated as easily as ambient pressures.