$MEX group (relevant if RUNTYP=MEX) This group governs a search for the lowest energy on the 3N-7 dimensional "seam" of intersection of two different electronic potential energy surfaces. Such Minimum Energy Crossing Points are important for processes such as spin- orbit coupling that involve transfer from one surface to another, and thus are analogous to transition states on a single surface. The present program requires that the two surfaces differ in spin quantum number, or space symmetry, or both. Analytic gradients are used in the search. In case the two potential surfaces have identical spin and space symmetry, this kind of intersection point is referred to as a Conical Intersection. See $CONICL using RUNTYP=CONICAL instead. SCF1, SCF2 = define the molecular wavefunction types, possibly in conjunction with the usual MPLEVL and DFTTYP keywords. MULT1, MULT2 = give the spin multiplicity of the states. Permissible combinations of wavefunctions are RHF with ROHF/UHF ROHF with ROHF UHF with UHF as well as their MP2 and DFT counterparts, and GVB with ROHF/UHF MCSCF with MCSCF (CISTEP=ALDET or GUGA only) NSTEP = maximum number of search steps (default=50) STPSZ = Step size during the search (default = 0.1D+00) NRDMOS = Initial orbitals can be read in = 0 No initial orbitals (default) = 1 Read in orbitals for first state (in $VEC1) = 2 Read in orbitals for second state (in $VEC2) = 3 Read in orbitals for both ($VEC1 and $VEC2) NMOS1 = Number of orbitals for first state's $VEC1. NMOS2 = Number of orbitals for second state's $VEC2. NPRT = Printing orbitals = 0 No orbital printed out except at the first geometry (default) = 1 Orbitals are printed each geometry. If MCSCF is used, CI expansions are also printed. Finer control of the convergence criterion: TDE = energy difference between two states (default = 1.0D-05) TDXMAX = maximum displacement of coordinates (default = 2.0D-03) TDXRMS = root mean square displacement (default = 1.5D-03) TGMAX = maximum of effective gradient between the two states (default = 5.0D-04) TGRMS = root mean square effective gradient tolerance (default = 3.0D-04) =========================================================== Usage notes: 1. Normally $CONTRL will not give SCFTYP or MULT keywords. SCF1 and SCF2 can be given in any order. The combinations permitted ensure roughly equal sophistication in the treatment of electron correlation. 2. After reading $MEX, SCFTYP and MULT will be set to the more complex of the two choices, which is considered to be RHF < ROHF < UHF < GVB < MCSCF. This permits the $SCF input defining a GVB wavefunction to be read and tested for correctness, in a GVB+ROHF run. Since only one SCFTYP is stored while reading the input, you might need to provide some keywords that are normally set by default for the other (such as ensuring DIIS is selected in $SCF if either of the states is UHF). 3. It is safest by far to prepare and read $VEC1 and $VEC2 groups so that you know what electronic states you start with. It is a good idea to regenerate both states at the end of the MEX search, to be sure that they remain as you began. 4. It is your responsibility to make sure that the states have a different space symmetry, or a different spin symmetry (or both). That is why note 3 is so important. 5. $GRAD1 and/or $GRAD2 groups containing gradients may be given to speed up the first geometry of the MEX search. 6. The search is even trickier than a saddle point search, for it involves the peaks and valleys of BOTH surfaces being generated. Starting geometries may be guessed as lying between the minima of the two surfaces, but the lowest energy on the crossing seam may turn out to be somewhere else. Be prepared to restart! 7. The procedure is a Newton-Raphson search, conducted in Cartesian coordinates, with a Lagrange multiplier imposing the constraint of equal energy upon the two states. The hessian matrices in the search are guessed at, and subjected to BFGS updates. Internal coordinates will be printed (for monitoring purposes) if you define $ZMAT, but the stepper operates in Cartesian coordinates only. No geometry constraints can be applied, apart from the point group in $DATA. A good paper to read about this kind of search is A.Farazdel, M.Dupuis J.Comput.Chem. 12, 276-282(1991) ===========================================================

generated on 7/7/2017