Given a set of PubMed IDs (PMIDs), you can use this converter to obtain the corresponding Pub Med Central IDs (PMCIDs) or the NIHMS IDs if a PMCID has not yet been assigned.
Anyone submitting an application, proposal or report to the NIH must include the PMC reference number (PMCID) when citing applicable manuscripts that they author or that arise from their NIH-funded research. Here's how to do it:
For papers published more than 3 months before an application, proposal and report is submitted:
For papers in press, or published within 3 months of when an application, proposal or report is submitted:
Source: http://publicaccess.nih.gov/include-pmcid-citations.htm
PMCID (PubMed Central reference number): The PMCID links to full-text articles in PubMed Central. Your article will eventually get a PMCID when the process is complete.
PMID (PubMed reference number): The PMID links to abstracts in PubMed. PMIDs have nothing to do with the NIH Public Access Policy.