The field is chemistry. We had a postdoc in our group who made three members of a series of compounds. He collected data, analyzed it, and wrote a paper (several other members of the group were included as co-authors for their contributions to various analyses).
The postdoc then left the group to take up a junior faculty position in another university.
Subsequently other members of the group (including co-authors of the original paper) synthesized further members of the original series, collected data, analyzed it and are in the process of writing another paper.
The topic of discussion is whether the original postdoc should be included as a co-author?
On the one hand, he did not make the samples, analyze them, or directly contribute to the second paper. All of the work happened after he had left the group. But on the other hand, the second paper is very much a follow-up to the original work, and would probably not have been possible without the work he put in on the project.
Should he be recognized in some way (co-authorship? acknowledgement?) or not?