Relations based on physical chemistry? (6,9)

I believe the answer is:

carnal knowledge

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'relations' is the definition.
(synonyms)

'on physical chemistry?' is the wordplay.
'on' says to put letters next to each other.
'physical' becomes 'carnal' (similar in meaning).
'chemistry?' becomes 'knowledge' (chemistry is a kind of knowledge).
'carnal' put next to 'knowledge' is 'CARNAL KNOWLEDGE'.

'based' is the link.
This may not be right. It may be part of another bit of the clue.

Can you help me to learn more?

(Another definition for carnal knowledge that I've seen is " It".)

I've seen this clue in The Independent.

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