Initial Guess Orbitals for Molecules Containing Transition Metals

For transition-metal-containing systems, particularly organometallics, you can often obtain superior results by improving the initial guess wave function. Jaguar automatically generates high-quality initial guesses for transition-metal-containing compounds; if you supply the program with information about the charges and spins of the “fragments” in the compounds, it uses that information when generating the guess. Here, a fragment is defined as either a collection of one or more transition metals that are bonded together, or one or more non-transition-metal atoms bonded together. Put another way, each fragment is simply a group of atoms that would be bonded together even if all bonds between transition metal atoms and non-transition-metal atoms were broken. Typically, the system is broken into ligand fragments and transition metal fragments, or adsorbate fragments and cluster fragments. For example, for ferrocene, the iron atom is one fragment, and the two cyclopentadienyl ligands are two additional fragments.

To supply Jaguar with information on charges and spins for its high-quality initial guess for a transition-metal-containing system, you need to edit the input file, either from the Edit Job dialog box (which you open by choosing Edit from the Settings button) or from a terminal window. First, add the following lines to the bottom of the input file:

&atomic
atom   formal   multip
&

(The exact number of spaces between words does not matter.)

Fill in information for each fragment under the headings atom, formal, and multip. You should add a single line for each fragment with a formal charge or a non-singlet spin multiplicity. The first entry in the line (under the heading atom) should be the atom label of any atom in the fragment. The next entry (under the heading formal, and separated from the first entry by one or more spaces) should be the formal charge of the entire fragment. The third entry (under the heading multip) should be the spin multiplicity of the fragment. If C1 is in one ring of ferrocene and C6 is in the other ring, then the following atomic section could be used to help generate the initial guess:

atom   formal   multip
Fe      +2       1
C1      -1       1
C6      -1       1
&

Fragments with no formal charge and singlet spin (water, for example) do not need to be listed in the atomic section, because Jaguar assumes a default formal charge of 0 and multiplicity of 1 for each fragment. Note, however, that any charge or spin multiplicity settings in the atomic section must be compatible with any settings for overall charge and spin specified by the molchg and multip keywords in the gen section. For more information about the atomic section, see The atomic Section of the Jaguar Input File.

After saving the input file with the atomic section, you can run it in Jaguar in the usual manner. You do not need to set iguess, because Jaguar will choose the most appropriate guess for the system under study.

If you have an antiferromagnetic system, the standard transition-metal initial guesses do not work. For an antiferromagnetic system containing two metal atoms that are not bonded, you can use a 2spin column in the atomic section to set up the initial guess. When the metals are within bonding distance, or when there are more than two metals, you should assign the metal atoms to separate fragments using the frag column of the atomic section. Finally, add formal and 2spin values in the atomic section.

Transition-metal systems can have multiple states based on different occupations of the d orbitals. If this is the case, the initial guess routine prints the possible states, and by default continues with the first state. However, this state might not be the lowest state. You should run calculations on all the possible states in turn to locate the true ground state. You can select states by setting the istate keyword in the gen section to the index of the state listed in the output from hfig. An example of this output is given in Output From a Standard HF Calculation. If you want to examine the coefficients of the MOs to see which state is which, set ip105=7.