I submitted a manuscript to a journal and recently I received the reviews. We have many reviewers assigned to the review process. One of the reviewers is asking us to perform some calculations that are beyond our knowledge. None of the co-authors know how to carry out such computations. All the other referees gave very good reviews but I'm not sure how to deal with this issue. Should we be honest in our answer to him or is there another way around? Is there a way we can avoid withdrawing our manuscript because of this request by this reviewer? I appreciate any advice in this regard.
I would suggest to the editor that it might be appropriate for future work and a future paper. You don't have to say what you say here and you don't have to follow up in the future unless you choose to.
But, I wouldn't withdraw the paper. Perhaps the editor will agree with you and perhaps not, but the other reviewers think it is worth publishing, so I'd guess you are fine. If you get rejected then reconsider.
The reviewer judgements don't need to be unanimous in most places. The editor weighs the evidence and decides.
Generally the reviewers can only give recommendations, they don't make the decision. I have seen many revisions that didn't do everything the reviewers asked for. The editor will have to balance things and decide whether the paper is good enough without this, which it may well be if a number of other reviewers liked it.
Normally you'd need to response to this and the editor will decide whether this is fine or send it to reviewers again. Now indeed "it would be a good idea but we are not able to do it" isn't really a very convincing response. You may actually get away with this (publication decisions are always something of a random process) but then you may not. Can't you argue that it's generally to much to do and goes beyond the scope of the paper, or another reason why it isn't such a good idea?
Another possibility, if what the reviewer suggests makes really good sense, is to look around whether at your university (or among people you or your co-authors know) could be an expert, maybe in a different department (not sure what this is about but just as an example it could be a mathematician), who has the skills to do this? You can then add a co-author to the revision, this is usually fine.
Do the suggested computations make sense? In other words, is the reviewer's suggestion valid? If they make sense or indeed, is a standard practice in your field, then you are missing a key piece of work. Otherwise, provide justifications/reasons why such computations do not help with your research aim. For example, 'Thank you for the suggested computations. However, the computations aim to shed light on X. However, our focus is on Y'. Alternatively, you can say, 'our evaluation on page-z includes a discussion on X. Hence, we do not believe such computations will provide further insights into X'.