You cannot simply choose to file MFS in order to get the PTC.
The IRS instructions for form 8962 (https://www.irs.gov/pub/irs-pdf/i8962.pdf) state the following:
"You can take the PTC for 2018 if you meet the conditions under (1) and (2) below (page 2)...
2.You are an applicable taxpayer for 2018. To be an applicable taxpayer, you must meet all of the following requirements.....c.If you were married at the end of 2018, generally you must file a joint return[emphasis mine]. However, filing a separate return from your spouse will not disqualify you from being an applicable taxpayer if you meet certain requirements described under Married taxpayers, later.(page 3)"
And what are the "certain requirements"? They are listed on page 5 under "Married Taxpayers". Basically, you have to satisfy either "Exception 1—Certain married persons living apart" or "Exception 2—Victim of domestic abuse or spousal abandonment." Most taxpayers don't meet either exception. What is clear is that you just can't choose to file separately in order to try to take the PTC.
