I have a fully converged DFT calculation (using Q-Chem 6.0) that used a def2-SVP basis set. I am trying to use the converged wave function result from the def2-SVP calculation as a starting point for a def2-TZVP calculation, without the def2-SVP calculation having to go through another complete SCF calculation to get an (already existing) converged wave function. I cannot find a way to do this in the Q-Chem manual. The only thing I find is the “dual-basis approximation” in Section 4.7.1 of the manual that requires the def2-SVP calculation to be re-run before the def2-TZVP guess is generated.

Is there any way to have Q-Chem project the existing def2-SVP orbitals into a starting set for the def2-TZVP calculation and not require the def2-SVP calculation to be redone?

What you need is basis-set projection, keyword BASIS2 in $rem.

Thanks for your reply.

That is the section of the manual I referenced. The example given requires the smaller basis set SCF be run to completion before the larger basis set run can be performed.

I don’t see information provided on how to use the files from an already converged smaller basis set run, from a separate job, to be used in the projection without a complete SCF job being run in the smaller basis set.

Is there an example of using BASIS2 using files from a previously run SCF of the smaller basis set as the input for the larger basis set run without having to rerun the smaller basis set job?

Just be careful because “dual basis” (a perturbative correction amounting to one Roothaan step in the larger basis set, following convergence in the smaller one) is different from “basis-set projection” (using small basis solution as a guess but fully converging the large basis). Both methods use the BASIS2 keyword. For what you want to do, you would have needed to save the small (def2-SVP) scratch files:

```
qchem -save small.in small.out small.scr
```

which would put the scratch files in $QCSCRATCH/small.scr. The “small.in” is a normal SCF with def2-SVP. To make sure you don’t accidentally overwrite the converged small-basis scratch files, I suggest

```
cp -r $QCSCRATCH/small.scr $QCSCRATCH/large.scr
```

Then run

```
qchem -save large.in large.out large.scr
```

where “large.in” has the following:

```
$rem
scf_guess read
basis def2-TZVP ! "target" (large) basis
basis2 def2-SVP ! small basis from previous calculation
... (etc) ...
$end
```

You should notice 2 SCF cycles in the small basis, which is the minimum number to guarantee that F and P are consistent. Then there’s a basis-projection step, may be a bit slow as I’m not sure how well parallelized that is as compared to the SCF cycles. Then the SCF cycles in the large basis should begin.

Thanks John. I’ll give it a try tomorrow.