In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Pain was less severe in Groups R4 and R6 than in the control and R2 groups ( P < 0 . 001 ) .
### Output Text:
Pain was less severe in Groups R4 and R6 than in the control and R2 groups ( P < 0 . 001 ) . 