Pulay residual subspace

The step stores positions and gradients, oldest first, and drops pairs past memory. The coefficients c solve

min || sum_i c_i g_i ||    subject to sum_i c_i = 1

which is the bordered Gram system of Pulay, Chem. Phys. Lett. 73, 393 (1980). Wood and Zunger, J. Phys. A, 1343 (1985), name the residual form. No DOI is attached for that line.

One stored pair, or a Gram matrix with no usable pivot, moves by -istep * g. Two or more pairs move to sum_i c_i x_i. Neither branch line searches on the oracle value, and neither applies a kinetic preconditioner. Control::maxmove or Solver::set_atom_maxmove caps the displacement the same way the other direct steps do.

Method::diis keeps 6 pairs. The C selector is RGMIN_DIIS (22). A control memory of 0 also means 6. ABI minor 30.