Is it wise to use ECPs on light atoms for electronic structure calculations?

So, I have a *big* organic molecule consisting of $\ce{H}$, $\ce{C}$, $\ce{O}$, and $\ce{N}$ atoms and the goal is to find its equilibrium geometry and US/Vis spectrum. The molecule is so big that I was thinking about reducing the complexity of the calculations by using effective core potentials (say, Stuttgart RLC) on *light* atoms ($\ce{C}$, $\ce{O}$, $\ce{N}$). However, I'm not sure how trustworthy the results would be comparing to using full-electron bases, thus, I'm looking for some reviews if there are any.




I think you will find that the time savings are minimal. Modern integral screening will eliminate virtually all inter-atom core-core integrals anyway and if you only have a 1s core then the cost of evaluating intra-atom core-valence integrals will be comparable to evaluating the ECP-core interaction. 


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If you want to make use of symmetry and reasonable parallelization for your TDDFT, you may want to try out [ADF](http://www.scm.com/ADF/).


ADF doesn't use ECPs, but you can freeze the core electrons.


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The ECP scheme will not bring substantial savings. If the molecule is really large (1000 atoms), you should better think of more efficient approximations, namely RI methods, RI-J, RI-JK or RIJCOSX. You can find their efficient implementation in [ORCA package](https://orcaforum.cec.mpg.de/).


In addition, I would suggest using some more recent basis sets, namely the Alrichs def2.


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