EOM-CCSD can also be used as the excited-state method to find minimum energy crossing points (MECPs). See Section 9.8.3 for more general information about MECP calculations.
The potential energy surface crossing optimization procedure finds energy minima of crossing seams. On the seam, the potential surfaces are degenerate in the subspace perpendicular to the plane defined by two vectors: the gradient difference
and the derivative coupling
At this time Q-Chem is unable to locate crossing minima for states which have non-zero derivative coupling. Fortunately, this does not occur often. Minima on the seams of conical intersections of states of different multiplicity can be found as their derivative coupling is zero. Minima on the seams of intersections of states of different point group symmetry can be located as well.
To run a PES crossing minimization, CCSD and EOM-CCSD methods must be employed for the ground and excited state calculations respectively.
Note: MECP optimization is only available for methods with analytic gradients. Finite-difference evaluation of two gradients is not possible.
Note: When performing coupled-cluster-based MECP calculations, one must use the XOPT_STATE_1 and XOPT_STATE_2 keywords and not MECP_STATE1 and MECP_STATE2.
Note: The spin can only be specified when using closed-shell RHF references. In the case of open-shell references all states are treated together, see also EE_STATES. For example, in spin-flip calculations use spin = 0 regardless of what is the actual multiplicity of the target state.
$molecule 1 1 N1 N2 N1 rnn N3 N2 rnn N1 annn rnn=1.46 annn=70.0 $end $rem JOBTYPE opt METHOD eom-ccsd BASIS 6-31g EE_SINGLETS [0,2,0,2] C2v point group symmetry XOPT_STATE_1 [0,4,1] 1B2 low spin state XOPT_STATE_2 [0,2,2] 2A2 low spin state XOPT_SEAM_ONLY true Find the seam only GEOM_OPT_TOL_GRADIENT 100 $end $opt CONSTRAINT Set constraints on the N-N bond lengths stre 1 2 1.46 stre 2 3 1.46 ENDCONSTRAINT $end @@@ $molecule READ $end $rem JOBTYPE opt Optimize the intersection seam METHOD eom-ccsd BASIS 6-31g EE_SINGLETS [0,2,0,2] XOPT_STATE_1 [0,4,1] XOPT_STATE_2 [0,2,2] GEOM_OPT_TOL_GRADIENT 30 $end
$molecule -1 1 N1 O2 N1 rno O3 N1 rno O2 aono rno = 1.3040 aono = 106.7 $end $rem JOBTYPE opt Optimize the intersection seam UNRESTRICTED true METHOD eom-ccsd N_FROZEN_CORE 0 BASIS 6-31g IP_STATES [1,0,1,0] C2v point group symmetry EOM_FAKE_IPEA 1 XOPT_STATE_1 [0,1,1] 1A1 low spin state XOPT_STATE_2 [0,3,1] 1B1 low spin state GEOM_OPT_TOL_GRADIENT 30 Tighten gradient tolerance CCMAN2 false $END