How to respond to a reviewer's comment when he is asking for experimental data but your paper is based on theoretical calculations? We submitted a research paper in physics.
In this paper, we take structural parameters from the experimental work and then perform simulations using density functional theory and present our results. No one has performed similar calculations or experiments before through which I could validate my results. One of the reviewer’s comments is:

Main problem of this work is a lack of the experimental data that could validate relevant calculations and which would be very important especially in actual case, at least from my point of view. Why?

My question is how should I respond to this comment?
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.