Prediction of Born-Oppenheimer Interatomic Forces Using Orbital-Free Density Functional Theory with Approximate Kinetic Energy Functionals*

Rapid calculation of Born-Oppenheimer forces is essential for driving the so called quantum region of a multi-scale molecular dynamics (MD) simulation. The orbital-free (OF) DFT approach is appealing but has proven difficult to implement because of the challenge of constructing reliable orbital-free approximations to the kinetic energy functional. To be maximally useful for multi-scale simulations, an OF-KE functional must be local (i.e. one-point). In the face of these difficulties, we demonstrate that there is a way forward. By requiring only that the approximate functional deliver high-quality forces, by exploiting the ``conjointness'' hypothesis of Lee, Lee, and Parr, by enforcing a basic positivity constraint, and by parameterizing to a carefully selected, small set of molecules we are able to generate a OF-KE functional that does a good job of describing various H$_q$Si$_m$O$_n$ clusters as well as CO and H$_2$O (providing encouraging evidence of transferability).