How to respond to a reviewer's comment when he is asking for experimental data but your paper is based on theoretical calculations?
Essentially, the reviewer is saying, you cannot/should not propose a new theory in a vacuum. One of two approaches apply:

Do you have a new theoretical approach that will explain observations that existing theoretical approaches cannot yet explain? Demonstrate this categorically by taking the experimental results modeled with the existing theories to model with your new theory. It seems that you argue that this is not the case for your theory. So ...

Do you have a new theoretical approach that points to new insights not yet measured by any existing experiments? State concisely what experimental measurements will be needed to validate your theory. You infer that this is your case.


Being somewhat aware of DFT and its applications as well as being experimentally inclined, I must say tangentially that I cannot trust that absolutely no experimental results (e.g. from spectroscopy) exist anywhere that can in some way validate your new theory, even in part. Alternatively, I cannot trust that your theory has absolutely no relevance to direct new experimental approaches in order to validate it (e.g. again spectroscopy).
In summary ... Einstein's Nobel prize was for a new theory that explained oddities in experimental observations (the photoelectric effect). Einstein's theories otherwise were put forward with proposals for experiments that might test their validity. You are being asked to do one or the other as well.