  We derive analytic energy gradients of the driven similarity renormalization
group (DSRG) multireference second-order perturbation theory (MRPT2) using the
method of Lagrange multipliers. In the Lagrangian, we impose constraints for a
complete-active-space self-consistent-field reference wave function and the
semicanonical orthonormal molecular orbitals. Solving the associated Lagrange
multipliers is found to share the same asymptotic scaling of a single
DSRG-MRPT2 energy computation. A pilot implementation of the DSRG-MRPT2
analytic gradients is used to optimize the geometry of the singlet and triplet
states of p-benzyne. The equilibrium bond lengths and angles are similar to
those computed via other MRPT2s and Mukherjee's multireference coupled cluster
theory. An approximate DSRG-MRPT2 method that neglects the contributions of
three-body density cumulant is found to introduce negligible errors in the
geometry of p-benzyne, lending itself to a promising low-cost approach for
molecular geometry optimizations using large active spaces.
